fix(docs): restore callout styling and improve highlight block visibility (#4956)

A recent just-the-docs update [broke callout block
styling](https://google.github.io/osv.dev/post-v1-query/#:~:text=dev/v1/query%22-,The%20API,-has%20a%20response)
on the docs pages.
This PR adds a custom override to restore callouts to their original
styles.

Also improved highlight block readability to match [the one applied in
osv-scanner docs](google/osv-scanner#2106) a
while back.

(Warning and Highlight blocks are not used anywhere in the osv.dev docs
yet, but this should help ensure the styles are ready for future
updates.)

---------

Co-authored-by: Rex P <106129829+another-rex@users.noreply.github.com>

Author: ashmod

docs/_sass/color_schemes/_variables.scss

@@ -21,7 +21,8 @@ $green-000: #41d693 !default;
 $green-100: #11b584 !default;
 $green-200: #009c7b !default;
 $green-300: #026e57 !default;
-$yellow-000: #ffeb82 !default;
+// Override the default yellow 000 for better visibility
+$yellow-000: #5f4f00;
 $yellow-100: #fadf50 !default;
 $yellow-200: #f7d12e !default;
 $yellow-300: #e7af06 !default;

docs/_sass/color_schemes/custom_dark.scss

@@ -15,7 +15,7 @@ $sidebar-color: $new-body-background-color; //Replacing default $grey-dk-300
 $base-button-color: $grey-dk-250;
 $btn-primary-color: $blue-200;
 $code-background-color:#202020; // Important to match with .highlight background and .highlight.err background-color 
-$code-linenumber-color: #447fcf; // Important to match with .highlight.nf color
+$code-linenumber-color: #f5f6fa; // Important to match with .highlight.nf color
 $feedback-color: darken($sidebar-color, 3%);
 $table-background-color: lighten($new-body-background-color, 5%); // Replacing default $grey-dk-250
 $search-background-color: $new-body-background-color; // Replacing default $grey-dk-250

docs/_sass/custom/custom.scss

@@ -0,0 +1,13 @@
+// Fixes callout styling regression introduced in just-the-docs v0.11.0 which
+// replaced the colored background fill with an all-sides border outline.
+// These overrides restore the left-border + colored-background style.
+
+@mixin callout-fix($bg-color, $border-color) {
+  border: none;
+  border-left: $border-radius solid $border-color;
+  background: rgba($bg-color, 0.2);
+}
+
+p.note, blockquote.note                { @include callout-fix($purple-000, $purple-300); }
+p.warning, blockquote.warning          { @include callout-fix($red-000, $red-300); }
+p.highlight, blockquote.highlight      { @include callout-fix($yellow-000, $yellow-300); }

docs/_sass/highlight/native.scss

@@ -131,7 +131,7 @@
     }
 
     .nc {
-      color: #447fcf;
+      color: #f5f6fa;
       text-decoration: underline;
     }
 
@@ -152,15 +152,15 @@
     }
 
     .nf {
-      color: #447fcf;
+      color: #f5f6fa;
     }
 
     .nl {
       color: #d0d0d0;
     }
 
     .nn {
-      color: #447fcf;
+      color: #f5f6fa;
       text-decoration: underline;
     }