css: Tidy up the card style

- remove the drop shadow
- clean up mixin references

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2513>

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

@@ -40,6 +40,7 @@ $button_mix_factor: 5%;
 
 // cards
 $card_bg_color: if($variant == 'light', darken($bg_color, 5%), lighten($bg_color, 2%));
+$card_outer_borders_color: transparentize($fg_color, 0.98);
 
 // notifications
 $bubble_buttons_color: if($variant == 'light', darken($bg_color, 12%), lighten($bg_color, 10%));

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

@@ -310,7 +310,7 @@
 @mixin card($flat: false) {
   border-radius: $base_border_radius;
   margin: $base_margin;
-  box-shadow: 0 1px 1px 0 $shadow_color;
+  box-shadow: inset 0 0 0 1px $card_outer_borders_color;
 
   @if $flat {
     @include button(undecorated);

data/theme/gnome-shell-sass/_high-contrast-colors.scss

@@ -38,6 +38,7 @@ $button_mix_factor: 10%;
 
 // cards
 $card_bg_color: if($variant == 'light', darken($bg_color, 12%), lighten($bg_color, 12%));
+$card_outer_borders_color: $card_bg_color;
 
 // notifications
 $bubble_buttons_color: if($variant == 'light', darken($bg_color, 22%), lighten($bg_color, 31%));

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

@@ -15,7 +15,7 @@
 
 /* today button (the date) */
 .datemenu-today-button {
-  @include card($flat: true);
+  @include card(flat);
   padding: $base_padding * 1.5;
 
   // weekday label