Fix code blocks "indent" without highlight.js

The `.hljs` class added by highlight.js adds a `display: block` rule
which makes `padding` apply correctly (left padding on all lines,
not just the first one).
Make sure that rule is applied even if highlight.js isn't run.

src/theme/css/chrome.css

@@ -260,6 +260,7 @@ pre > .buttons button {
     }
 }
 pre > code {
+    display: block;
     padding: 1rem;
 }