css: change code block background for branding

Change the codeblock background from the blue tint to use a grey from our new
branding colors.

Signed-off-by: Mike Szczys <mike@golioth.io>

Author: szczys

src/css/custom.css

@@ -92,6 +92,10 @@ button [class^="lightToggleIcon"]:not(:hover) {
   --ifm-hero-text-color: white;
 }
 
+pre code {
+  background-color: var(--golioth-color-cod-gray);
+}
+
 .docusaurus-highlight-code-line {
   background-color: rgb(72, 77, 91);
   display: block;