Firefox-UI-Fix/src/compatibility/_os.scss

/*= Windows 7, 8 - Tab Bar Background *****************************************/
@include OS($win7, $win8) {
  /* Header Color */
  :root:not([lwtheme-image]):-moz-lwtheme {
    background-color: var(--lwt-accent-color) !important;
  }
}
@include OS($win7) {
  #TabsToolbar:-moz-lwtheme {
    color: var(--lwt-text-color) !important;
  }

  /* Remove Aero */
  #TabsToolbar {
    /* Original: radial-gradient(eclipse at bottom, rgba(255, 255, 255, 0.8), rgba(255, 255, 255, 0.5) 80%, transparent) */
    background-image: unset !important;
  }
}
@include OS($win8) {
  #navigator-toolbox:-moz-lwtheme {
    color: var(--lwt-text-color) !important;
  }
}

/*= Windows 7, 8 - Menu Active Color =========================================*/
@include OS($win7, $win8) {
  menupopup > menu,
  menupopup > menuitem,
  #context-navigation > menuitem {
    border: 1px solid transparent; /* Need reduce 2px at menu */
  }

  #main-menubar > menu[open="true"],
  #main-menubar > menu[_moz-menuactive="true"] {
    background-color: -moz-menuhover !important; /* Make to original */
  }
  :root:-moz-lwtheme #main-menubar > menu[open="true"],
  :root:-moz-lwtheme #main-menubar > menu[_moz-menuactive="true"] {
    color: inherit !important; /* Original: -moz-menubarhovertext */
    background-color: color-mix(in srgb, currentColor 20%, transparent) !important; /* Original: -moz-menuhover */
  }

  menulist > menupopup > menuitem[_moz-menuactive="true"],
  menulist > menupopup > menu[_moz-menuactive="true"] {
    background-color: highlight !important;
    color: highlighttext !important;
  }

  #PlacesToolbar menu,
  #PlacesToolbar menuitem,
  #BMB_bookmarksPopup menu,
  #BMB_bookmarksPopup menuitem {
    border: none !important; /* Remove border */
  }
  #PlacesToolbar menu:not([disabled], :active)[_moz-menuactive="true"],
  #PlacesToolbar menuitem:not([disabled], :active)[_moz-menuactive="true"],
  #BMB_bookmarksPopup menu:not([disabled], :active)[_moz-menuactive="true"],
  #BMB_bookmarksPopup menuitem:not([disabled], :active)[_moz-menuactive="true"] {
    background-color: var(--button-hover-bgcolor) !important;
  }
}

@include OS($win7) {
  menu[_moz-menuactive="true"],
  menuitem[_moz-menuactive="true"] {
    background-color: color-mix(in srgb, -moz-menuhover 5%, transparent) !important;
    border-color: color-mix(in srgb, -moz-menuhover 60%, transparent) !important;
  }
  menu[_moz-menuactive="true"][disabled="disabled"],
  menuitem[_moz-menuactive="true"][disabled="true"] {
    background-color: color-mix(in srgb, currentColor 5%, transparent) !important;
    border-color: color-mix(in srgb, currentColor 60%, transparent) !important;
  }

  /* Remove text shadow */
  :root:-moz-lwtheme #toolbar-menubar {
    text-shadow: unset !important;
    /* Original: 0 0 .5em white, 0 0 .5em white, 0 1px 0 rgba(255, 255, 255, .4) */
  }

  /* Remove Color */
  :root:-moz-lwtheme #main-menubar:not(:-moz-window-inactive) {
    background-color: unset !important; /* Original: rgba(255, 255, 255, .5) */
    color: unset !important; /* Original: black */
  }
}
@include OS($win8) {
  menu[_moz-menuactive="true"],
  menuitem[_moz-menuactive="true"] {
    background-color: color-mix(in srgb, -moz-menuhover 17%, transparent) !important;
    border-color: color-mix(in srgb, -moz-menuhover 80%, transparent) !important;
  }
  menu[_moz-menuactive="true"][disabled="disabled"],
  menuitem[_moz-menuactive="true"][disabled="true"] {
    background-color: color-mix(in srgb, currentColor 17%, transparent) !important;
    border-color: color-mix(in srgb, currentColor 80%, transparent) !important;
  }
}

@include OS($linux) {
  /*= Linux - Global Menubar Active Color ====================================*/
  #main-menubar > menu[open="true"],
  #main-menubar > menu[_moz-menuactive="true"] {
    color: inherit !important; /* Original: -moz-menubarhovertext */
    background-color: color-mix(in srgb, currentColor 20%, transparent) !important; /* Original: -moz-menuhover */
  }

  /*= Linux - Titlebar button at lwtheme =====================================*/
  .titlebar-button:-moz-lwtheme {
    appearance: auto !important;
  }
  .titlebar-min:-moz-lwtheme,
  .titlebar-max:-moz-lwtheme,
  .titlebar-restore:-moz-lwtheme,
  .titlebar-close:-moz-lwtheme {
    list-style-image: none !important;
  }
  .titlebar-button:-moz-lwtheme:hover,
  .titlebar-button:-moz-lwtheme:hover:active {
    background-color: unset !important;
    color: unset !important;
  }
}

/*= Linux - Light System Default Theme's Selected Tab ========================*/
@include OS($linux) {
  @include Light {
    /* Because of
      #TabsToolbar:not([brighttext]) #tabbrowser-tabs:not([noshadowfortests]) .tabbrowser-tab:is([visuallyselected=true], [multiselected]) > .tab-stack > .tab-background {
        border: 1px solid var(--tab-line-color, rgba(128,128,142,0.9));
        box-shadow: 0 0 4px rgba(128,128,142,0.5);
      }
     */
    #tabbrowser-tabs:not([noshadowfortests])
      .tabbrowser-tab:is([visuallyselected="true"], [multiselected])
      > .tab-stack
      > .tab-background:not(:-moz-lwtheme) {
      box-shadow: 0 0 4px rgba(128, 128, 142, 0.5) !important;
    }
    #TabsToolbar:not([brighttext])
      #tabbrowser-tabs:not([noshadowfortests])
      .tabbrowser-tab:is([visuallyselected="true"], [multiselected])
      > .tab-stack
      > .tab-background:not(:-moz-lwtheme) {
      box-shadow: 0 0 1px var(--tab-line-color, rgba(128, 128, 142, 0.9)), 0 0 4px rgba(128, 128, 142, 0.5) !important;
    }
  }
}