You’re on the right track! However, please note that the CustomCSS field is only for adding styles to the main element. It can’t be used to style sub-elements, which is why your selectors might disappear from the field after refreshing the page or deselecting and reselecting the element.
To achieve what you want, you should either add the custom CSS to the head of the specific page or, if it’s something you’ll use across multiple pages, to the site-wide CSS.
To add the code to a single page, simply select the page from the dropdown menu, click the gear icon, then click on “Custom Code” and add your CSS to the head section.
To apply this rule site-wide, go to the top bar, click “More” (right next to Copilot), then click “Custom Code” and add the CSS to the head.