feat(plugins/code): improvements (#891) [skip ci]

source/templates/classic/style.css

@@ -1254,12 +1254,18 @@
   .snippet .body {
     padding-left: 12px;
   }
+  .snippet .language-diff {
+    font-size: 77%;
+  }
   .snippet.additions {
     color: #336543;
   }
   .snippet.deletions {
     color: #9A5256;
   }
+  .snippet .token.prefix.unchanged {
+    visibility: hidden;
+  }
 
 /* Markdown and syntax highlighting */
   .markdown b, .markdown i {