style: Fix some high contrast papercuts

- add HC style to slider
- add HC style to Quick settings submenu header
- fix border issue with flat card style

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7320
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3165>

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

@@ -159,6 +159,7 @@ stage {
 
 %card_flat {
   @extend %card_common;
+  border: 1px solid transparent !important; // override of the button style needed here
   @include button(undecorated, $style: flat);
   &:hover { @include button(hover, $style: flat);}
   &:active { @include button(active, $style: flat);}

data/theme/gnome-shell-sass/widgets/_calendar.scss

@@ -1,7 +1,6 @@
 /* Date/Time Menu */
 
 // overall menu
-
 #calendarArea {
   padding: $base_margin;
 }

data/theme/gnome-shell-sass/widgets/_quick-settings.scss

@@ -122,6 +122,14 @@
         background-color: $selected_bg_color;
         color: $selected_fg_color;
       }
+
+      // draw hc outline
+      @if $contrast == 'high' {
+        @include draw_hc_inset();
+        &.active {
+          @include draw_hc_inset($no_inset: true);
+        }
+      }
     }
 
     & .title {

data/theme/gnome-shell-sass/widgets/_slider.scss

@@ -19,4 +19,9 @@ $slider_size: $scalable_icon_size;
   -slider-handle-radius: $slider_size * 0.5; // half the size of the size
   -slider-handle-border-width: 0;
   -slider-handle-border-color: transparent; // because 0 width
+
+    // hc style
+    @if $contrast == 'high' {
+      -barlevel-background-color: transparentize($fg_color, 0.6);
+    }
 }