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

// Drawing mixins

// generic drawing of more complex things

@function draw_widget_edge($c:$borders_edge) {
// 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; }
}

// entries

@mixin entry($t, $fc:$selected_bg_color, $edge: $borders_edge) {
//
// Entries drawing function
//
// $t: entry type
// $fc: focus color
// $edge: set to none to not draw the bottom edge or specify a color to not use the default one
//
// possible $t values:
// normal, focus, insensitive
//

  @if $t==normal {
    background-color: $base_color;
    border-color: $borders_color;

  }
  @if $t==focus {
    border-color: if($fc==$selected_bg_color,
              $selected_borders_color,
              darken($fc,35%));
    box-shadow: inset 0 0 0 1px $fc;
  }
  @if $t==hover { }
  @if $t==insensitive {
    color: $insensitive_fg_color;
    border-color: $insensitive_bg_color;
    box-shadow: none;
  }
}

// buttons

@function draw_border_color ($c) {
  //
  // colored buttons want the border form the base color
  //
  @return if($variant == 'light', darken($c, 18%), darken($c, 4%));
}

@function draw_text_shadow_color ($tc:$fg_color, $bg:$bg_color) {
//
// calculate the color of text shadows
//
// $tc is the text color
// $bg is the background color
//
  $lbg: lightness($bg)/100%;
  @if lightness($tc)<50% { @return rgba(255,255,255,$lbg/($lbg*1.3)); }
  @else { @return rgba(0,0,0,1-$lbg*0.8); }
}

@function draw_button_hilight_color($c) {
//
// calculate the right top highlight color for buttons
//
// $c: base color;
//
  @if lightness($c)>90% { @return white; }
  @else if lightness($c)>80% { @return rgba(255,255,255, 0.7); }
  @else if lightness($c)>50% { @return rgba(255,255,255, 0.5); }
  @else if lightness($c)>40% { @return rgba(255,255,255, 0.3); }
  @else { @return rgba(255,255,255, 0.1); }
}

@mixin draw_button_text_shadow ($tc:$fg_color, $bg:$bg_color) {
//
// helper function for the text emboss effect
//
// $tc is the optional text color, not the shadow color
//
// TODO: this functions needs a way to deal with special cases
//

  $shadow: draw_text_shadow_color($tc, $bg);

  @if lightness($tc)<50% {
    text-shadow: 0 1px $shadow;
    icon-shadow: 0 1px $shadow;
  }
  @else {
    text-shadow: 0 -1px $shadow;
    icon-shadow: 0 -1px $shadow;
  }
}

@mixin button($t, $c:$bg_color, $tc:$fg_color, $edge: $borders_edge, $shadow: $shadow_color) {
//
// Button drawing function
//
// $t:    button type,
// $c:    base button color for colored* types
// $tc:   optional text color for colored* types
// $edge: set to none to not draw the bottom edge or specify a color to not
//        use the default one
// $shadow: set to none to not draw the drop shadow or specify a color to not
//          use the default one
//
// 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
//

  $hilight_color: draw_button_hilight_color($c);
  $button_edge: if($edge == none, none, draw_widget_edge($edge));
  $blank_edge: if($edge == none, none, draw_widget_edge(transparentize($edge,1)));
  $button_shadow: if($shadow == none, none, 0 1px 1px 0 $shadow);

  // normal button
  @if $t==normal {
    color: $tc;
    background-color: lighten($c, 3%);
    border-color: draw_border_color($c);
    @include draw_shadows($button_shadow);
    // box-shadow: 0 1px 1px 0 rgba(0,0,0,0.1);
    text-shadow: 0 1px $text_shadow_color;
    icon-shadow: 0 1px $text_shadow_color;
  }

  // focused button
  @if $t==focus {
    color: $tc;
    text-shadow: 0 1px $text_shadow_color;
    icon-shadow: 0 1px $text_shadow_color;
    box-shadow: inset 0 0 0 2px transparentize($selected_bg_color, 0.4);
    //border-color: $selected_bg_color;
  }

  // hover button
  @else if $t==hover {
    color: $tc;
    background-color: lighten($c, if($variant == 'light', 8%, 5%));
    border-color: if($variant == 'light', draw_border_color(lighten($c, 7%)), draw_border_color($c));
    @include draw_shadows($button_shadow);
    text-shadow: 0 1px $text_shadow_color;
    icon-shadow: 0 1px $text_shadow_color;
  }

  // active button
  @else if $t==active {
    color: $tc;
    background-color: darken($c,3%);
    border-color: draw_border_color(if($variant == 'light', $c, darken($c,7%)));
    text-shadow: none;
    icon-shadow: none;
    box-shadow: none;
  }

  // insensitive button
  @else if $t==insensitive {
    color: $insensitive_fg_color;
    border-color: $insensitive_borders_color;
    background-color: $insensitive_bg_color;
    box-shadow: none;
    text-shadow: none;
    icon-shadow: none;
  }

  // reset
  @else if $t==undecorated {
    border-color: transparent;
    background-color: transparent;
    background-image: none;
    @include draw_shadows(inset 0 1px rgba(255,255,255,0),$blank_edge);
    text-shadow: none;
    icon-shadow: none;
  }
}

// overview icons
@mixin overview-icon($color) {
  .overview-icon {
    @extend %icon_tile;
    color: $color;
  }

  &:hover,
  &:selected {
    .overview-icon {
      background-color: transparentize($color, .9);
    }
  }

  &:focus {
    .overview-icon {
      background-color: transparentize($color, .7);
      // border-color: $selected_bg_color;
    }
  }

  &:drop {
    .overview-icon {
      border: 2px solid $selected_bg_color; //already 2px transparent so no jumping
      background-color: transparentize($selected_bg_color, .8);
    }
  }

  &:active,
  &:checked {
    .overview-icon {
      background-color: transparentize(darken($osd_bg_color, 10%), .5);
    }
  }
}