Examine source code of Dark Material You

{
  "manifest_version": 2,
  "version": "1.0",
  "name": "dark material you",
  "theme": {
    "images": {},
    "properties": {},
    "colors": {
      "toolbar": "rgb(60, 60, 60)",
      "toolbar_text": "rgb(227, 227, 227)",
      "frame": "rgb(31, 32, 32)",
      "tab_background_text": "rgb(199, 199, 199)",
      "toolbar_field": "rgb(40, 40, 40)",
      "toolbar_field_text": "rgb(227, 227, 227)",
      "tab_line": "rgba(227, 227, 227, 0)",
      "popup": "rgb(31, 31, 31)",
      "popup_text": "rgb(227, 227, 227)",
      "icons": "rgb(199, 199, 199)",
      "popup_border": "rgb(44, 44, 44)",
      "popup_highlight": "rgb(80, 80, 79)",
      "sidebar_highlight_text": "rgb(255, 255, 255)",
      "sidebar_highlight": "rgb(0, 74, 118)",
      "sidebar_text": "rgb(227, 227, 227)",
      "sidebar": "rgb(60, 60, 60)",
      "toolbar_bottom_separator": "rgb(69, 71, 70)",
      "toolbar_field_border_focus": "rgb(101, 157, 246)",
      "toolbar_field_focus": "rgb(60, 60, 60)",
      "tab_loading": "rgba(227, 227, 227, 0)"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{8124d0c3-8d66-406b-924f-98a205192c6f}"
    }
  }
}