Selected Tab Color UserStyle

github-selected-tab-color-uso.user.css

/* ==UserStyle==
@name        GitHub Selected Tab Color (uso)
@version     v1.0.10
@description Add a custom selected tab color to GitHub
@namespace   github.com/StylishThemes
@author      StylishThemes (https://github.com/StylishThemes)
@homepageURL https://github.com/StylishThemes/GitHub-Selected-Tab-Color
@supportURL  https://github.com/StylishThemes/GitHub-Selected-Tab-Color/issues
@updateURL   https://raw.githubusercontent.com/StylishThemes/GitHub-Selected-Tab-Color/master/github-selected-tab-color.user.css
@license     CC-BY-SA-4.0
@advanced color base-color 'Tab color' #4183C4
==/UserStyle== */
@-moz-document domain("github.com") {
  /* AGENT_SHEET */
  body .pagehead-tabs-item.selected,
  body .reponav-item.selected,
  body .btn-link.selected,
  body .tabnav-tab.selected,
  body .tabnav-pr .tabnav-tab.selected, /* issue #9 */
  body #graph_data .tab.selected {
    border-top-width: 3px !important;
    border-top-color: /*[[base-color]]*/ !important;
  }
  body .repo-filterer .repo-filter.filter-selected,
  body .filter-selected,
  body .underline-nav-item.selected,
  body .sub-nav ul a.active {
    border-bottom-color: /*[[base-color]]*/ !important;
  }
  body .menu-item.selected:before {
    background-color: /*[[base-color]]*/ !important;
  }
  body .js-current .standalone a,
  body .sidebar-module .disable > a {
    border-left-color: /*[[base-color]]*/ !important;
  }
  /* reset GitHub Dark shifting tabs up */
  body .reponav-item, .pagehead-tabs-item {
    position: static !important;
  }
}

github-selected-tab-color.user.css

/*
==UserStyle==
@name        GitHub Selected Tab Color (stylus)
@version     v1.0.10
@description Add a custom selected tab color to GitHub
@namespace   github.com/StylishThemes
@author      StylishThemes (https://github.com/StylishThemes)
@homepageURL https://github.com/StylishThemes/GitHub-Selected-Tab-Color
@supportURL  https://github.com/StylishThemes/GitHub-Selected-Tab-Color/issues
@license     CC-BY-SA-4.0
@advanced color base-color 'Tab color' #4183C4
@preprocessor stylus
==/UserStyle==
*/
@-moz-document domain("github.com") {
  /* AGENT_SHEET */
  body .pagehead-tabs-item.selected,
  body .reponav-item.selected,
  body .btn-link.selected,
  body .tabnav-tab.selected,
  body .tabnav-pr .tabnav-tab.selected, /* issue #9 */
  body #graph_data .tab.selected {
    border-top-width: 3px !important;
    border-top-color: base-color !important;
  }
  body .repo-filterer .repo-filter.filter-selected,
  body .filter-selected,
  body .underline-nav-item.selected,
  body .sub-nav ul a.active {
    border-bottom-color: base-color !important;
  }
  body .menu-item.selected:before {
    background-color: base-color !important;
  }
  body .js-current .standalone a,
  body .sidebar-module .disable>a {
    border-left-color: base-color !important;
  }
  /* reset GitHub Dark shifting tabs up */
  body .reponav-item, .pagehead-tabs-item {
    position: static !important;
  }
}

test-dropdown.user.css

/* ==UserStyle==
@name test
@namespace tester
@version 0.1.0
@license unlicense
@advanced dropdown browser "Your browser" {
  fx "Firefox" <<<EOT
color: red; EOT;
  cr "Chrome" <<<EOT
color: green; EOT;
}
==/UserStyle== */
@-moz-document domain("stackoverflow.com") {
  body {
    /*[[browser]]*/
  }
}

See openstyles/stylus#134 (comment)