CSS variant for the Let's Go web book adding dark mode support and using system fonts.

main.css

:root {
    /* Text Width */
    --text-width: 860px;

    /* Typography */
    --main-font: system-ui, sans-serif;
    --code-font:
        ui-monospace, "Cascadia Code", "Source Code Pro", Menlo, Consolas,
        "DejaVu Sans Mono", monospace;

    /* Text Colors */
    --text-primary: #283c46;
    --text-muted: #818181;
    --link-color: #007bb6;

    /* Background Colors */
    --bg-contrast: #f9f9f9;
    --bg: #ffffff;

    /* Borders */
    --border-color: #ededed;

    /* Code Syntax Highlighting */
    --code-keyword: #a82255;
    --code-string: #008075;
    --code-number: #5642a6;

    /* Aside Backgrounds */
    --bg-important: #fbf4f7;
    --bg-note: #f2f8fb;
    --bg-hint: #f2f9f8;

    /* Miscellaneous */
    --silver: silver;
}

/* Dark mode - inverted lightness */
@media (prefers-color-scheme: dark) {
    :root {
        /* Text Colors */
        --text-primary: #d7e3ec;
        --text-muted: #9e9e9e;
        --link-color: #4da6db;

        /* Background Colors */
        --bg-contrast: #1a1a1a;
        --bg: #0a0a0a;

        /* Borders */
        --border-color: #333333;

        /* Code Syntax Highlighting */
        --code-keyword: #e85b88;
        --code-string: #4db8a8;
        --code-number: #9b8dd9;

        /* Aside Backgrounds */
        --bg-important: #2a1f24;
        --bg-note: #1a2428;
        --bg-hint: #1a2827;

        /* Miscellaneous */
        --silver: #606060;
    }
}

/*! normalize.css v8.0.1 | MIT License | github.com/necolas/normalize.css */
/* Document ========================================================================== */
/* 1. Correct the line height in all browsers. 2. Prevent adjustments of font size after orientation changes in iOS. */
html {
    line-height: 1.15; /* 1 */
    text-size-adjust: 100%; /* 2 */
}

/* Sections ========================================================================== */
/* Remove the margin in all browsers. */
body {
    margin: 0;
}

/* Render the `main` element consistently in IE. */
main {
    display: block;
}

/* Correct the font size and margin on `h1` elements within `section` and `article` contexts in Chrome, Firefox, and Safari. */
h1 {
    font-size: 2em;
    margin: 0.67em 0;
}

/* Grouping content ========================================================================== */
/* 1. Add the correct box sizing in Firefox. 2. Show the overflow in Edge and IE. */
hr {
    box-sizing: content-box; /* 1 */
    height: 0; /* 1 */
    overflow: visible; /* 2 */
}

/* 1. Correct the inheritance and scaling of font size in all browsers. 2. Correct the odd `em` font sizing in all browsers. */
pre {
    font-family: var(--code-font); /* 1 */
    font-size: 1em; /* 2 */
}

/* Text-level semantics ========================================================================== */
/* Remove the gray background on active links in IE 10. */
a {
    background-color: transparent;
}

/* 1. Remove the bottom border in Chrome 57- 2. Add the correct text decoration in Chrome, Edge, IE, Opera, and Safari. */
abbr[title] {
    border-bottom: none; /* 1 */
    text-decoration: underline; /* 2 */
    text-decoration: underline dotted; /* 2 */
}

/* Add the correct font weight in Chrome, Edge, and Safari. */
b,
strong {
    font-weight: bolder;
}

/* 1. Correct the inheritance and scaling of font size in all browsers. 2. Correct the odd `em` font sizing in all browsers. */
code,
kbd,
samp {
    font-family: var(--code-font); /* 1 */
    font-size: 1em; /* 2 */
}

/* Add the correct font size in all browsers. */
small {
    font-size: 80%;
}

/* Prevent `sub` and `sup` elements from affecting the line height in all browsers. */
sub,
sup {
    font-size: 75%;
    line-height: 0;
    position: relative;
    vertical-align: baseline;
}

sub {
    bottom: -0.25em;
}

sup {
    top: -0.5em;
}

/* Embedded content ========================================================================== */
/* Remove the border on images inside links in IE 10. */
img {
    border-style: none;
}

/* Forms ========================================================================== */
/* 1. Change the font styles in all browsers. 2. Remove the margin in Firefox and Safari. */
button,
input,
optgroup,
select,
textarea {
    font-family: inherit; /* 1 */
    font-size: 100%; /* 1 */
    line-height: 1.15; /* 1 */
    margin: 0; /* 2 */
}

/* Show the overflow in IE. 1. Show the overflow in Edge. */
button,
input {
    /* 1 */
    overflow: visible;
}

button,
select {
    /* 1 */
    text-transform: none;
}

button,
[type="button"],
[type="reset"],
[type="submit"] {
    appearance: button;
}

button::-moz-focus-inner,
[type="button"]::-moz-focus-inner,
[type="reset"]::-moz-focus-inner,
[type="submit"]::-moz-focus-inner {
    border-style: none;
    padding: 0;
}

button:-moz-focusring,
[type="button"]:-moz-focusring,
[type="reset"]:-moz-focusring,
[type="submit"]:-moz-focusring {
    outline: 1px dotted ButtonText;
}

fieldset {
    padding: 0.35em 0.75em 0.625em;
}

legend {
    box-sizing: border-box; /* 1 */
    color: inherit; /* 2 */
    display: table; /* 1 */
    max-width: 100%; /* 1 */
    padding: 0; /* 3 */
    white-space: normal; /* 1 */
}

progress {
    vertical-align: baseline;
}

textarea {
    overflow: auto;
}

[type="checkbox"],
[type="radio"] {
    box-sizing: border-box; /* 1 */
    padding: 0; /* 2 */
}

[type="number"]::inner-spin-button,
[type="number"]::outer-spin-button {
    height: auto;
}

[type="search"] {
    appearance: textfield; /* 1 */
    outline-offset: -2px; /* 2 */
}

[type="search"]::search-decoration {
    appearance: none;
}

::file-upload-button {
    appearance: button; /* 1 */
    font: inherit; /* 2 */
}

details {
    display: block;
}

summary {
    display: list-item;
}

template {
    display: none;
}

[hidden] {
    display: none;
}

html,
body,
p,
ol,
ul,
li,
dl,
dt,
dd,
blockquote,
figure,
fieldset,
legend,
textarea,
pre,
iframe,
hr,
h1,
h2,
h3,
h4,
h5,
h6 {
    margin: 0;
    padding: 0;
}

h1,
h2,
h3,
h4,
h5,
h6 {
    font-size: 100%;
    font-weight: normal;
}

ul {
    list-style: none;
}

button,
input,
select,
textarea {
    margin: 0;
}

html {
    box-sizing: border-box;
    overflow-y: scroll;
}

*,
*:before,
*:after {
    box-sizing: inherit;
}

img,
embed,
iframe,
object,
audio,
video {
    height: auto;
    max-width: 100%;
}

iframe {
    border: 0;
}

table {
    border-collapse: collapse;
    border-spacing: 0;
}

td,
th {
    padding: 0;
    text-align: left;
}

img {
    vertical-align: middle;
}
/*rgba(0,0,0,0.0235)*/
/*Used for .bash samp*/
body {
    font-family: var(--main-font);
    font-size: 20px;
    line-height: 1.5;
    color: var(--text-primary);
    background-color: var(--bg);
}

a,
a:visited {
    color: var(--link-color);
    text-decoration: none;
}
a:hover,
a:visited:hover {
    text-decoration: underline;
}

h1#front-matter {
    display: none;
}

/*contents*/
nav ol li {
    font-weight: 700;
    list-style-type: disc;
}

nav ol li.indent {
    font-weight: 400;
    margin-left: 24px;
    list-style-type: circle;
}

.text code,
.text table,
.text pre,
.text samp,
.text figcaption {
    font-family: var(--code-font);
}
.text blockquote {
    font-style: italic;
    border-left: solid 4px var(--border-color);
    padding-left: 1em;
}
.text blockquote code {
    font-style: normal;
}
.text figure.code {
    font-size: 16px;
    background-color: var(--bg-contrast);
    border: solid 1px var(--border-color);
    overflow-x: auto;
}
.text figure.code figcaption {
    padding: 14px 1em;
    border-bottom: solid 1px var(--border-color);
    margin-bottom: -3px;
}
.text figure.code pre {
    padding: 1em;
    tab-size: 4;
    -moz-tab-size: 4;
    -o-tab-size: 4;
}
.text figure.bash {
    background-color: var(--text-primary);
    color: var(--bg);
    border: none;
}
.text figure.bash samp {
    color: var(--silver);
}
.text figure,
.text table {
    margin: 30px 0;
}
.text p,
.text ul,
.text ol,
.text aside {
    margin: 20px 0;
}
.text em code {
    font-style: normal;
}
.text h1,
.text h2 {
    font-weight: 700;
    font-size: 45px;
    border-bottom: solid 1px var(--border-color);
    padding-bottom: 15px;
    margin-bottom: 30px;
}
.text h3 {
    font-weight: 700;
    font-size: 30px;
    margin: 45px 0 20px;
}
.text h4 {
    font-weight: 700;
    font-size: 22px;
    margin: 30px 0 20px;
}
.text h3 + h4 {
    margin-top: 20px;
}
.text table {
    width: 100%;
}
.text ul,
.text ol {
    padding-left: 1em;
}
.text ul li,
.text ol li {
    padding-left: 0.25em;
}
.text ul {
    list-style-type: disc;
}
.text li + li {
    margin-top: 5px;
}
.text code {
    color: var(--code-keyword);
    font-size: 19px;
    padding: 0px 1px;
    white-space: nowrap;
}
.text a code {
    color: var(--link-color);
}
.text figure.img {
    border: solid 1px var(--border-color);
    box-shadow: 1px 1px 2px 0px var(--border-color);
    -moz-box-shadow: 1px 1px 2px 0px var(--border-color);
    box-shadow: 1px 1px 2px 0px var(--border-color);
}
.text aside {
    padding: 1px 1em;
    border-left-style: solid;
    border-left-width: 6px;
}
.text aside.important {
    background-color: var(--bg-important);
    border-color: var(--code-keyword);
}
.text aside.note {
    background-color: var(--bg-note);
    border-color: var(--link-color);
}
.text aside.hint {
    background-color: var(--bg-hint);
    border-color: var(--code-string);
}
.text blockquote {
    font-style: italic;
}
.text aside {
    border-left-style: solid;
    border-left-width: 6px;
}
.text table {
    width: 100%;
}
.text table td,
.text table th {
    font-size: 16px;
}
.text table td code,
.text table th code {
    font-size: 16px;
    white-space: nowrap;
}
.text table td {
    padding: 10px 9px;
    border: solid 1px var(--border-color);
    vertical-align: top;
}
.text table th {
    padding: 10px 9px 9px;
    background-color: var(--text-primary);
    border-left: solid 1px var(--text-primary);
    border-right: solid 1px var(--text-primary);
    border-bottom: solid 1px var(--text-primary);
    color: var(--bg);
}
.text table tr:nth-child(2n) {
    background-color: var(--bg-contrast);
}
.text table span {
    white-space: nowrap;
}
.text .chapter {
    color: var(--text-muted);
    font-size: 16px;
    color: var(--text-muted);
    margin-bottom: 5px;
}
.text ul ul {
    list-style-type: circle;
}
.text ul ul li {
    margin-top: 5px;
}
.text hr {
    margin-top: 60px;
    border: none;
    border-top: solid 1px var(--border-color);
}

figure.code .c {
    color: var(--text-muted);
}

/* Comment */
figure.code .ch {
    color: var(--text-muted);
}

/* Comment.Hashbang */
figure.code .cm {
    color: var(--text-muted);
}

/* Comment.Multiline */
figure.code .cp {
    color: var(--text-muted);
}

/* Comment.Preproc */
figure.code .cpf {
    color: var(--text-muted);
}

/* Comment.PreprocFile */
figure.code .c1 {
    color: var(--text-muted);
}

/* Comment.Single */
figure.code .cs {
    color: var(--text-muted);
}

/* Comment.Special */
figure.code .s {
    color: var(--code-string);
}

/* Literal.String */
figure.code .sa {
    color: var(--code-string);
}

/* Literal.String.Affix */
figure.code .sb {
    color: var(--code-string);
}

/* Literal.String.Backtick */
figure.code .sc {
    color: var(--code-string);
}

/* Literal.String.Char */
figure.code .dl {
    color: var(--code-string);
}

/* Literal.String.Delimiter */
figure.code .sd {
    color: var(--code-string);
}

/* Literal.String.Doc */
figure.code .s2 {
    color: var(--code-string);
}

/* Literal.String.Double */
figure.code .se {
    color: var(--code-string);
}

/* Literal.String.Escape */
figure.code .sh {
    color: var(--code-string);
}

/* Literal.String.Heredoc */
figure.code .si {
    color: var(--code-string);
}

/* Literal.String.Interpol */
figure.code .sx {
    color: var(--code-string);
}

/* Literal.String.Other */
figure.code .sr {
    color: var(--code-string);
}

/* Literal.String.Regex */
figure.code .s1 {
    color: var(--code-string);
}

/* Literal.String.Single */
figure.code .ss {
    color: var(--code-string);
}

/* Literal.String.Symbol */
figure.code .m {
    color: var(--code-number);
}

/* Literal.Number */
figure.code .il {
    color: var(--code-number);
}

/* Literal.Number.Integer.Long */
figure.code .mb {
    color: var(--code-number);
}

/* Literal.Number.Bin */
figure.code .mf {
    color: var(--code-number);
}

/* Literal.Number.Float */
figure.code .mh {
    color: var(--code-number);
}

/* Literal.Number.Hex */
figure.code .mi {
    color: var(--code-number);
}

/* Literal.Number.Integer */
figure.code .mo {
    color: var(--code-number);
}

/* Literal.Number.Oct */
figure.code .k {
    color: var(--code-keyword);
}

/* Keyword */
figure.code .kc {
    color: var(--code-keyword);
}

/* Keyword.Constant */
figure.code .kd {
    color: var(--code-keyword);
}

/* Keyword.Declaration */
figure.code .kn {
    color: var(--code-keyword);
}

/* Keyword.Namespace */
figure.code .kp {
    color: var(--code-keyword);
}

/* Keyword.Pseudo */
figure.code .kr {
    color: var(--code-keyword);
}

/* Keyword.Reserved */
/*figure.code .kt { color: $rose;} /* Keyword.Type */
figure.code .o {
    color: var(--code-number);
}

/* Operator */
figure.code .ow {
    color: var(--code-number);
}

/* Operator.Word */
figure.code .na {
    color: var(--code-number);
}

/* Name.Attribute */
/*figure.code .nb { color: $rose;} /* Name.Builtin */
figure.code .nc {
    color: var(--code-number);
}

/* Name.Class */
figure.code .no {
    color: var(--code-number);
}

/* Name.Constant */
figure.code .nd {
    color: var(--code-number);
}

/* Name.Decorator */
figure.code .ni {
    color: var(--code-number);
}

/* Name.Entity */
figure.code .ne {
    color: var(--code-number);
}

/* Name.Exception */
figure.code .nl {
    color: var(--code-number);
}

/* Name.Label */
figure.code .nn {
    color: var(--code-number);
}

/* Name.Namespace */
figure.code .nt {
    color: var(--code-number);
}

/* Name.Tag */
figure.code .nv {
    color: var(--code-number);
}

/* Name.Variable */
figure.code .bp {
    color: var(--code-number);
}

/* Name.Builtin.Pseudo */
figure.code .fm {
    color: var(--code-number);
}

/* Name.Function.Magic */
figure.code .vc {
    color: var(--code-number);
}

/* Name.Variable.Class */
figure.code .vg {
    color: var(--code-number);
}

/* Name.Variable.Global */
figure.code .vi {
    color: var(--code-number);
}

/* Name.Variable.Instance */
figure.code .vm {
    color: var(--code-number);
}

/* Name.Variable.Magic */
.qa {
    margin: 30px 0;
}

.qa > div {
    background: var(--bg-contrast);
    border: solid 1px var(--border-color);
    padding-left: 1em;
    padding-right: 1em;
    clear: both;
}

.qa .a {
    border-top: solid 1px transparent;
}
.qa .a .hidden {
    display: none;
}

.qa ul {
    list-style-type: none;
    padding-left: 0;
}
.qa ul input {
    position: relative;
    left: -5px;
    top: -2px;
    vertical-align: middle;
    margin-right: 5px;
}

.show-answer {
    display: block;
    margin: 20px 0;
}

.wrapper {
    width: 100%;
    max-width: var(--text-width);
    margin-left: auto;
    margin-right: auto;
}

main {
    margin-top: 60px;
    margin-bottom: 60px;
    min-height: calc(100vh - 75px - 75px - 120px);
}

header,
footer {
    background-color: var(--bg-contrast);
    padding: 25px 0;
    font-size: 16px;
    color: var(--text-muted);
}

header {
    border-bottom: solid 1px var(--border-color);
}
header .wrapper {
    display: grid;
    grid-template-columns: 1fr auto;
}

footer {
    border-top: solid 1px var(--border-color);
}
footer .wrapper {
    display: grid;
    grid-template-columns: repeat(3, 80px);
    justify-content: space-between;
}
footer .wrapper div:nth-child(2) {
    text-align: center;
}
footer .wrapper div:last-child {
    text-align: right;
}

img[src="assets/img/cover.png"] {
    margin-bottom: 30px;
}

@media only screen and (max-width: 820px) {
    body {
        font-size: 18px;
    }
    .wrapper {
        padding-left: 30px;
        padding-right: 30px;
    }
    main {
        margin-top: 40px;
        margin-bottom: 40px;
    }
    .crumbs {
        display: none;
    }
    .text h1,
    .text h2 {
        font-size: 37px;
    }
}

Added an additional variable for adjusting the width of the text. 760px was a bit narrow for my liking.