Add selector for background colour for hljs

Author: WebCoder49

docs/interface/css/_index.md

@@ -8,5 +8,5 @@ title = 'Styling `code-input` elements with CSS'
 
 `code-input` elements can be styled like `textarea` elements in most cases; however, there are some exceptions:
 * The CSS variable `--padding` should be used rather than the property `padding` (e.g. `<code-input style="--padding: 10px;">...`)
-* Background colours set on `code-input` elements will not work with highlighters that set background colours themselves - use `(code-input's selector) pre[class*="language-"]` for Prism.js, `(code-input's selector) TODO` for highlight.js to target the highlighted element with higher specificity than the highlighter's theme.
+* Background colours set on `code-input` elements will not work with highlighters that set background colours themselves - use `(code-input's selector) pre[class*="language-"]` for Prism.js or `.hljs` for highlight.js to target the highlighted element with higher specificity than the highlighter's theme. You may also set the `background-color` of the code-input element for its appearance when its template is unregistered / there is no JavaScript.
 * For now, elements on top of `code-input` elements should have a CSS `z-index` at least 3 greater than the `code-input` element.