Merge branch '430055-consolidate-border-colors' into 'master'

Consolidate SCSS variables for border colors

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/137221



Merged-by: Artur Fedorov <afedorov@gitlab.com>
Approved-by: Dan Mizzi-Harris <dmizzi-harris@gitlab.com>
Approved-by: Laura Callahan <lmeckley@gitlab.com>
Approved-by: Artur Fedorov <afedorov@gitlab.com>
Reviewed-by: Dan Mizzi-Harris <dmizzi-harris@gitlab.com>
Reviewed-by: Laura Callahan <lmeckley@gitlab.com>
Reviewed-by: Scott de Jonge <sdejonge@gitlab.com>
Co-authored-by: sdejonge <sdejonge@gitlab.com>

app/assets/stylesheets/themes/dark_mode_overrides.scss

@@ -137,7 +137,6 @@
 
 aside.right-sidebar:not(.right-sidebar-merge-requests) {
   background-color: $gray-10;
-  border-left-color: $gray-50;
 }
 
 :root.gl-dark {

ee/app/assets/stylesheets/pages/issues.scss

@@ -21,8 +21,8 @@
     background: $white;
     margin: 16px -20px -20px;
     padding: 16px 20px;
-    border-top: 1px solid $border-gray-light;
-    border-bottom: 1px solid $border-gray-light;
+    border-top: 1px solid $border-color;
+    border-bottom: 1px solid $border-color;
 
     a:hover {
       color: $btn-white-active;