theme-github.css

/* use variables when about:config variable userChrome.theme-github is true */
@media -moz-pref("userChrome.theme-github") {

  :root,
  html,
  body {
    --md-text-primary: #e6edf3;
    --md-text-secondary: #7d8590;
    --md-accent-color: #2f81f7;
    --md-background-color-0: #0d1117;
    --md-background-color-50: #161b22;
    --md-background-color-100: #24292f;
    --md-menu-background-color: #161b22;
    --md-menu-background-color-hover: #6e768166;
    --md-menu-border-color: #31363d;
    --md-content-separator-color: #30363db3;
    --md-selection-text-color: var(--md-white);
    --md-selection-background-color: #1f6febb3;
    --md-text-on-accent: var(--md-white);
  }
}