Document caret/placeholder theme behaviour

Author: WebCoder49

docs/interface/css/_index.md

@@ -10,5 +10,6 @@ title = 'Styling `code-input` elements with CSS'
 * The CSS variable `--padding` should be used rather than the property `padding` (e.g. `<code-input style="--padding: 10px;">...`), or `--padding-left`, `--padding-right`, `--padding-top` and `--padding-bottom` instead of the CSS properties of the same names. For technical reasons, the value must have a unit (i.e. `0px`, not `0`).
 * 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.
+* The caret and placeholder colour by default follow and give good contrast with the highlighted theme. Setting a CSS rule (with a specificity higher than one element and one class, for good backwards compatibility) for `color` and/or `caret-color` properties on the code-input element will override this behaviour.
 
 Please do **not** use `className` in JavaScript referring to code-input elements, because the code-input library needs to add its own classes to code-input elements for easier progressive enhancement. You can, however, use `classList` and `style` as much as you want - it will make your code cleaner anyway.