Fix styling of example blocks in dark mode

Author: HT154

src/supplemental-ui/css/site-extra.css

@@ -125,3 +125,10 @@ code {
 .toc.sidebar .toc-menu h3 {
     padding-left: calc(0.5rem + 2px);
 }
+
+.doc .exampleblock > .content, .doc details.result > .content {
+    background: var(--color-syntax-bg);
+    border: .25rem solid #5d5d5d;
+    border-radius: .5rem;
+    padding: .75rem;
+}