gnome-shell/data/theme/gnome-shell-sass/_drawing.scss

// Drawing mixins

// generic drawing of more complex things

@function draw_widget_edge($c:$outer_borders_color) {
// outer highlight "used" on most widgets
  @return 0 1px $c;
}

// provide font size in rem, with px fallback
@mixin fontsize($size: 24, $base: 16) {
  font-size: round($size) + pt;
  //font-size: ($size / $base) * 1rem;
}

@mixin draw_shadows($shadow1, $shadow2:none, $shadow3:none, $shadow4:none) {
//
// Helper function to stack up to 4 box-shadows;
//
  @if $shadow4!=none { box-shadow: $shadow1, $shadow2, $shadow3, $shadow4; }
  @else if $shadow3!=none { box-shadow: $shadow1, $shadow2, $shadow3; }
  @else if $shadow2!=none { box-shadow: $shadow1, $shadow2; }
  @else { box-shadow: $shadow1; }
}


// Text entries
@mixin entry($t, $c, $fc:$selected_bg_color) {
//
// Entries drawing function
//
// $t: entry type
// $c: text color, used to derive background color of entries
//
// possible $t values: normal, focus, insensitive
//
  transition-duration: 100ms;

  @if $t==normal {
    background-color: transparentize($c, 0.85);
    color: transparentize($c,0.3);

    @if $is_highcontrast {
      box-shadow: inset 0 0 0 1px $hc_inset_color;
    }
  }

  @if $t==focus {
    background-color: mix(transparentize($c, 0.75), $fc, 95%);
    box-shadow: inset 0 0 0 2px transparentize($fc, 0.3);
    color: $c;
    &:hover {}
  }

  @if $t==hover {
    background-color: transparentize($c, 0.75);
  }

  @if $t==insensitive {
    background-color:transparentize($c, 0.75);
    color: transparentize($c, 0.5);
  }
}

// On-screen Keyboard
@mixin keyboard_key($t, $c:$osd_bg_color, $tc:$osd_fg_color) {
//
// Keyboard key drawing function
//
// $t:    key type,
// $c:    base key color for colored* types
// $tc:   optional text color for colored* types
//
// possible $t values:
// normal, hover, active, insensitive, insensitive-active,
// backdrop, backdrop-active, backdrop-insensitive, backdrop-insensitive-active,
// osd, osd-hover, osd-active, osd-insensitive, osd-backdrop, undecorated
//

  // normal key
  @if $t==normal {
    color: $tc;
    background-color: lighten($c, 3%);
  }

  // focused key
  @if $t==focus {
    color: $tc;
    background-color: mix(lighten($c, 3%), $selected_bg_color, 90%);
    box-shadow: inset 0 0 0 2px transparentize($selected_bg_color, 0.4);
    &:hover {
      background-color: mix(lighten($c, 8%), $selected_bg_color, 90%);
      box-shadow: inset 0 0 0 2px transparentize($selected_bg_color, 0.3);
    }
    &:active {
      background-color: mix(lighten($c, 10%), $selected_bg_color, 90%);
      box-shadow: inset 0 0 0 2px transparentize($selected_bg_color, 0.3);
    }
  }

  // hover key
  @else if $t==hover {
    color: $tc;
    background-color: lighten($c, 7%);
  }

  // active key
  @else if $t==active {
    color: $tc;
    background-color: lighten($c, 10%);
  }

  // checked key
  @else if $t==checked {
    color: $tc;
    background-color: lighten($c, 15%);
  }

  // insensitive key
  @else if $t==insensitive {
    color: $insensitive_fg_color;
    background-color: $insensitive_bg_color;
  }

  // reset
  @else if $t==undecorated {
    background-color: transparent;
    background-image: none;
  }
}

//
// Button drawing function
//
// $t:    button type,
// $c:    base button colors, derived from fg_color
// $tc:    base button colors, derived from fg_color
//
// possible $t values:
// normal, hover, active, insensitive, insensitive-active,
// backdrop, backdrop-active, backdrop-insensitive, backdrop-insensitive-active,
// osd, osd-hover, osd-active, osd-insensitive, osd-backdrop, undecorated
//
// since buttons are all flat an borderless now the mixin is simpler

@mixin button($t, $tc:$fg_color, $c:$bg_color, $flat: false) {

  $button_bg_color: mix($tc, $c, $button_mix_factor);
  transition-duration: 100ms;

  // normal button
  @if $t==normal {
    color: $tc;
    background-color: $button_bg_color;
    @if $flat {
      background-color: transparent;
    }
    @if $is_highcontrast {
      box-shadow: inset 0 0 0 1px $hc_inset_color;
    }
  }

  // focused button
  @if $t==focus {
    color: $tc;
    background-color: mix($button_bg_color, $selected_bg_color, 90%);
    box-shadow: inset 0 0 0 2px transparentize($selected_bg_color, 0.4) !important;
    &:hover {
      background-color: mix(lighten($button_bg_color, 3%), $selected_bg_color, 90%);
      box-shadow: inset 0 0 0 2px transparentize($selected_bg_color, 0.3) !important;
    }
    &:active {
      background-color: mix(lighten($button_bg_color, 6%), $selected_bg_color, 90%);
      box-shadow: inset 0 0 0 2px transparentize($selected_bg_color, 0.3) !important;
    }
  }

  // hover button
  @else if $t==hover {
    color: $tc;
    background-color: if($variant == 'light', darken($button_bg_color, 3%), lighten($button_bg_color, 3%));

    @if $is_highcontrast == "true" {
      box-shadow: inset 0 0 0 1px lighten($button_inset_color, 3%);
      background-color: mix(lighten($button_bg_color, 3%), $button_inset_color, 10%);
    }
  }

  // active button
  @else if $t==active {
    color: $tc;
    background-color: if($variant == 'light', darken($button_bg_color, 6%), lighten($button_bg_color, 6%));
    @if $is_highcontrast == "true" {
      box-shadow: inset 0 0 0 1px lighten($button_inset_color, 6%);
      background-color: mix(lighten($button_bg_color, 6%), $button_inset_color, 10%);
    }
  }

  // checked button
  @else if $t==checked {
    color: $tc;
    background-color: if($variant == 'light', darken($button_bg_color, 9%), lighten($button_bg_color, 9%));
    @if $is_highcontrast == "true" {
      box-shadow: inset 0 0 0 1px lighten($button_inset_color, 9%);
      background-color: mix(lighten($button_bg_color, 9%), $button_inset_color, 10%);
    }
    &:hover { background-color: lighten($button_bg_color, 12%);}
    &:active { background-color: lighten($button_bg_color, 15%);}
  }

  // insensitive button
  @else if $t==insensitive {
    background-color: if($variant == 'light', transparentize($tc, .9), transparentize($tc, .95));
    color: if($variant == 'light', transparentize($tc, .6), transparentize($tc, .5));
  }

  // default/suggested button
  @else if $t==default {
    background-color: $selected_bg_color;
    color: $selected_fg_color;

    &:focus {
      box-shadow: inset 0 0 0 2px transparentize($selected_fg_color, .4) !important;
    }
    &:hover, &:focus {
      background-color: lighten($selected_bg_color, 5%);
      color: lighten($selected_fg_color, 5%);
    }
    &:active {
      background-color: darken($selected_bg_color, 7%);
      color: darken($selected_fg_color, 7%);
    }
    &:insensitive {
      @include button(insensitive);
      background-color: transparentize($selected_bg_color, .5);
      color: transparentize($selected_fg_color, .5);
    }
  }

  // reset
  @else if $t==undecorated {
    background-color: transparent;
    background-color: none;
    box-shadow: none;
    &:insensitive {
      @include button(insensitive);
      background-color: transparent;
    }
  }
}

// tile
@mixin tile_button($color, $flat: true) {
  @extend %tile;
  @if $flat {
    background-color: transparent;
  } @else {
    background-color: transparentize($color, .84);
    @if $is_highcontrast {
      box-shadow: inset 999px 0 0 0 transparentize($color, .2); // bit of a hack
    }
  }
  &:hover { background-color: transparentize($color, .9);}
  &:selected, &:focus {
    background-color: transparentize($color, .87);
    &:hover { background-color: transparentize($color, .84);}
    &:active { background-color: transparentize($color, .87);}
  }
  &:active { background-color: transparentize($color, .84);}
  &:outlined, &:checked {
    background-color: transparentize($color, .81);
    &:active { background-color: transparentize($color, .78);}
    &:hover { background-color: transparentize($color, .75);}
  }
  &:drop {
    border: 2px solid transparentize($selected_bg_color, .2); //already 2px transparent so no jumping
    background-color: transparentize($selected_bg_color, .8);
  }
}

// overview icon, dash, app grid
@mixin overview_icon($color, $flat: true) {
  transition-duration: 400ms;
  .overview-icon {
    @extend %tile;
  }
  @if $flat {
    .overview-icon { background-color: transparent;}
  } @else {
    .overview-icon { background-color: transparentize($color, .93); }
  }
  &:hover .overview-icon { background-color: transparentize($color, .87);}

  &:selected .overview-icon,
  &:focus .overview-icon {
    background-color: transparentize($color, .87);
    &:hover .overview-icon { background-color: transparentize($color, .84);}
    &:active .overview-icon { background-color: transparentize($color, .87);}
  }
  &:active .overview-icon { background-color: transparentize($color, .84);}
  &:outlined .overview-icon,
  &:checked .overview-icon {
    background-color: transparentize($color, .81);
    &:active .overview-icon { background-color: transparentize($color, .78);}
    &:hover .overview-icon { background-color: transparentize($color, .75);}
  }
  &:drop .overview-icon {
    border: 2px solid transparentize($selected_bg_color, .2); //already 2px transparent so no jumping
    background-color: transparentize($selected_bg_color, .8);
  }
}

// styling for elements within popovers that look like notifications
@mixin card($flat: false) {
  border-radius: $base_border_radius*1.5;
  margin: $base_margin;

  @if $flat {
    @include button(undecorated);
    box-shadow: none !important;
  } @else {
    @include button(normal);
  }
  &:hover {@include button(hover);}
  &:active {@include button(active);}
  &:focus {@include button(focus);}
  &:insensitive {
    @include button(insensitive);
    @if $flat {
     background-color: transparent;
    }
  }
}

// styling for all menuitems in popovers
@mixin menuitem($bg, $flat: true) {

  // lighten the background color always
  $bg: lighten($bg,5%);

  font-weight: normal;
  spacing: $base_padding;
  transition-duration: 100ms;
  padding: $base_padding*1.5 $base_padding*2;

  @if $flat {
    @include button(undecorated);
    box-shadow: none !important;
  } @else {
    @include button(normal, $c:$bg);
  }
  &:focus,
  &:hover {
    @include button(hover, $c:$bg);
  }
  &:active {@include button(active, $c:$bg);}
  &:checked {@include button(checked, $c:$bg);}
}

//
// Panel menu/button drawing function
//
// $bg: background color, derived from $panel_fg_color
// $fg: foreground color, also derived from $panel_fg_color
//
// $flat:               if true, the button is filled instead of transparent
// $highlighted_child:  if true, applies some special overrides for to a
//                      child element, see _panel.scss for details
// $child_class:        class name of the child element
//

@mixin panel_button($bg:$panel_fg_color, $fg:$panel_fg_color, $flat: true, $highlighted_child: false, $child_class:"") {

  transition-duration: 150ms;
  border: 3px solid transparent;
  border-radius: 99px;

  font-weight: bold;
  background-color: transparent;
  color: $fg;

  @if $flat {
    box-shadow: none;

    &:active, &:focus, &:checked {
      box-shadow: inset 0 0 0 100px transparentize($fg, 0.75);
      &:hover {
        box-shadow: inset 0 0 0 100px transparentize($fg, 0.65);
      }
    }

    &:hover {
      box-shadow: inset 0 0 0 100px transparentize($fg, 0.8);
    }

  } @else {
    box-shadow: inset 0 0 0 100px transparentize($bg, 0.2);

    &:active, &:focus, &:checked {
      box-shadow: inset 0 0 0 100px $bg;
      &:hover {
        box-shadow: inset 0 0 0 100px transparentize($bg, 0.05);
      }
    }

    &:hover {
      box-shadow: inset 0 0 0 100px transparentize($bg, 0.1);
    }
  }

  // some overrides to style a child element
  @if $highlighted_child {
    box-shadow: none !important;

    #{$child_class} {
      transition-duration: 150ms;
      border: 3px solid transparent;
      border-radius: 99px;
    }

    &:active, &:focus, &:checked {
      box-shadow: none !important;

      #{$child_class} {
        box-shadow: inset 0 0 0 100px transparentize($fg, 0.75);

        &:hover {
          box-shadow: inset 0 0 0 100px transparentize($fg, 0.65) !important;
        }
      }
    }

    &:hover {
      box-shadow: none !important;
      #{$child_class} {
        box-shadow: inset 0 0 0 100px transparentize($fg, 0.8);
      }
    }
  }
}