gnome-shell: Optimize contrast for warning, success and destructive texts

In practice the shell only use the warning color, but optimize them all
in case some extension relies on the other ones (yep, ubuntu-dock!)

LP: #2126731

Author: 3v1n0

gnome-shell/src/gnome-shell-sass/_yaru-colors.scss

@@ -1,3 +1,5 @@
+@import 'sass-utils';
+
 // Yaru custom definitions
 $base_hover_color: transparentize(white, 0.8);
 $base_active_color: transparentize(white, 0.75);
@@ -35,3 +37,19 @@ $osd_insensitive_bg_color: transparentize(mix($osd_fg_color, opacify($osd_bg_col
 $osd_insensitive_fg_color: mix($osd_fg_color, opacify($osd_bg_color, 1), 50%);
 $osd_borders_color: transparentize(black, 0.3);
 $osd_outer_borders_color: transparentize(white, 0.84);
+
+// Overrides defaults for contrast optimized ones.
+
+// colors for destructive elements
+$destructive_bg_color: optimize-contrast($destructive_fg_color, $red, $target: 5.5);
+$destructive_color: optimize-contrast($bg_color, $red, $target: 6.0);
+
+// colors for levelbars, entries, labels and infobars
+$success_bg_color: optimize-contrast($success_fg_color, $green, $target: 5.5);
+$success_color: optimize-contrast($bg_color, $green, $target: 6.0);
+
+$warning_bg_color: optimize-contrast($warning_fg_color, $yellow, $target: 5.5);
+$warning_color: optimize-contrast($bg_color, $yellow, $target: 6.0);
+
+$error_bg_color: optimize-contrast($error_fg_color, $red, $target: 5.5);
+$error_color: optimize-contrast($bg_color, $red, $target: 6.0);

gnome-shell/src/gnome-shell-sass/_yaru-default-colors.scss

@@ -4,6 +4,3 @@ $_base_color_light: $light_1; // Yaru: use our colors
 
 // Used for dash and other dark elements on light theme
 $yaru_borders_color_dark: lighten(desaturate(lighten($jet, 4%), 100%), 14%);
-
-// Overrides default
-$warning_bg_color: if($variant =='light', darken($yellow, 1%), $yellow);

gnome-shell/src/gnome-shell-sass/widgets/_osd.scss

@@ -1,5 +1,7 @@
 /* OSD */
 
+@import 'sass-utils';
+
 $osd_levelbar_height:6px;
 
 .osd-window {
@@ -27,7 +29,8 @@ $osd_levelbar_height:6px;
     -barlevel-height: $osd_levelbar_height;
     -barlevel-background-color: transparentize($osd_fg_color, if($variant=='light', 0.7, 0.9));
     -barlevel-active-background-color: $osd_fg_color;
-    -barlevel-overdrive-color: $destructive_color;
+    // Yaru: Optimize overdrive color for contrast
+    -barlevel-overdrive-color: optimize-contrast($osd_bg_color, $destructive_color, $target: 5.5);
     -barlevel-overdrive-separator-width: $base_padding * 0.5;
     &:ltr { margin-right: $base_padding; }
     &:rtl { margin-left: $base_padding; }