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

div#json, div#jfContent{
  margin: 0;
  padding: 20px;
  background: #191919;
  color: #C4BD97; 
  position: fixed;
  top:0;
  bottom:0;
  left:0;
  right:0;
  font-family: Consolas, mono;
  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;
}