style: Clean up color definitions and High Contrast

- clean out unused colors; add desaturated light color
- remove long unused drawing functions; add high contrast color mixin
- de-duplicate and move common colors to a default-colors stylesheet
- rework how HighContrast is defined; clean up HC cruft
- bring over named color defines and other colors from libadwaita
- change how panel colors are defined

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3097>

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

@@ -79,7 +79,7 @@ stage {
   text-align: center;
   color: inherit;
 
-  @if $is_highcontrast {
+  @if $contrast == 'high' {
     border-color: $hc_inset_color;
   }
 }
@@ -128,6 +128,7 @@ stage {
   border: 1px solid $card_shadow_border_color;
 }
 
+
 // normal entry style
 %entry_common {
   border-radius: $base_border_radius;
@@ -199,7 +200,7 @@ stage {
   padding: $base_padding $base_padding * 2;
   text-align: center;
 
-  @if $is_highcontrast {
+  @if $contrast == 'high' {
     background-color: black;
     border-color: $hc_inset_color;
   }
@@ -262,9 +263,10 @@ stage {
   border: 1px solid $osd_outer_borders_color;
   border-radius: $forced_circular_radius;
   padding: $base_padding * 2;
+  border: 2px solid transparent;
 
-  @if $is_highcontrast {
-    border: 2px solid $hc_inset_color;
+  @if $contrast == 'high' {
+    border-color: $hc_inset_color;
   }
 }