style: Adjust the tooltip style; fix inconsistency with screenshot UI

- remove screenshot tooltips using a style different from other tooltips
- tone down how dark they are
- fix missing background adjustment for high contrast

Fixes #6660

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2757>

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

@@ -167,6 +167,7 @@ stage {
 // tooltip
 %tooltip {
   background-color: transparentize(black, 0.1);
+  border: 1px solid transparentize($light_1, 0.9);
   color: $light_1;
 
   border-radius: 99px;
@@ -174,6 +175,7 @@ stage {
   text-align: center;
 
   @if $is_highcontrast {
+    background-color: black;
     border-color: $hc_inset_color;
   }
 }

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

@@ -195,10 +195,7 @@ $screenshot_ui_button_red: $error_color;
 }
 
 .screenshot-ui-tooltip {
-  color: $osd_fg_color;
-  background-color: $osd_bg_color;
-  border-radius: 99px;
-  padding: $base_padding $base_padding * 2;
+  @extend %tooltip;
   text-align: center;
   -y-offset: 24px;
 }