style: use consistent `pre` and `code` bg colors

See #1740

assets/scss/academic/_dark.scss

@@ -39,7 +39,7 @@ body.dark,
 }
 
 .dark pre {
-  background-color: rgb(68, 71, 90);
+  /* Match `pre` bg color above. */
   border-color: rgb(68, 71, 90);
 }
 

assets/scss/academic/_root.scss

@@ -168,12 +168,13 @@ pre,
 code {
   font-family: $sta-font-mono, monospace;
   color: #c7254e;
-  background-color: #f9f2f4;
+  /* Match bg of default highlight theme. */
+  background-color: rgb(248, 248, 248);
 }
 
 pre {
   margin: 0 0 1rem 0;
-  background-color: rgb(248, 248, 248); /* Match default highlight theme. */
+  /* Match bg of default highlight theme. */
   border-color: rgb(248, 248, 248);
   font-size: 0.7rem;
   border-radius: 4px;