Dark Theme for Firefox/Chrome add-ons JSON View (http://jsonview.com/) and JSON Formatter (https://github.com/callumlocke/json-formatter) by using the add-on Stylish (https://userstyles.org/)

userstyle_jsonview.css

/* ==UserStyle==
@name           JSONView Dark
@namespace      https://gist.github.com/lennybacon
@version        1.0.1
@description    Dark theme for JsonView Extension
@author         lennybacon
@homepageURL  https://gist.github.com/lennybacon/cb700cee27601e7473dd
@updateURL    https://gist.githubusercontent.com/lennybacon/cb700cee27601e7473dd/raw/7cfb8b250a8067122a0f138e4f00ab463b9d59d4/userstyle_jsonview.css
@license      CC-BY-SA-4.0
==/UserStyle== */

div#json,
div#jfContent {
    font-family: 'Fira Code', 'Cascadia Code', 'Consolas', 'DejaVu Sans Mono', Monaco, 'Lucida Console', monospace, monospace;
    margin: 0;
    padding: 20px;
    background: #191919;
    color: #C4BD97;
    position: fixed;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    font-size: 13px;
    z-index: 1;
    overflow-y: scroll;
}

div#jfContent .b,
div#jfContent .kvov.objProp {
    color: #C4BD97;
}

div.hovered {
    background: #191919;
}

div#json .string,
div#json .type-string,
div#jfContent .s {
    color: #FF8000;
}

div#jfContent .k {

    color: #8080FF;
}

div#json .num,
div#json .type-number,
div#jfContent .n {

    color: #FF00FF;
}
div#json .bool,
div#json .type-boolean,
div#jfContent .bl {
    color: #51A8FF;
}
div#json .null,
div#json .type-null,
div#jfContent .nl {
    color: #51A8FF;
}
div#json a {

    color: #8080FF;
}

div#error {
    margin: 0;
    padding: 0px;
    background: #FF0000;
    color: #FFF;
    position: fixed;
    z-index: 1000;
    bottom: 0;
    left: 0;
    right: 0;
    font-family: Consolas, mono;
    font-size: 13px;
}

div#error .errormessage {
    color: #FFF;
    font-family: Consolas, mono;
    font-size: 13px;
}

.prop {
    font-weight: normal!important;
}