update doc styles for 'implementation' sections (#14441)

diff --git a/dev/docs/styles.html b/dev/docs/styles.html
index 42fa609..8671389 100644
--- a/dev/docs/styles.html
+++ b/dev/docs/styles.html
@@ -133,12 +133,9 @@
     font-weight: 100;
   }
 
-  /* Attempt to de-emphasise the source code section */
-  section.summary.source-code {
-    opacity: 0.3;
-  }
-  section.summary.source-code:hover {
-    opacity: 0.8;
+  /* address a style issue with the background of code sections */
+  code.hljs {
+    background: inherit;
   }
 
   footer {