Only some action elements allow HTML and thus these are the only ones CSS styling can be applied to. Most of these are in Locize and a few are editable in the general action settings. These include:
Styling is added via CSS and ideally the classes using the Tailwind CSS framework. It allows plenty of customisation.
Custom html tags and styling might reduce the accessabilitiy and cross-device compatability the platform has been optimised for. Keep html tags to the basics (e.g. p, div, span, string, em, br, links, images) and use Tailwind CSS classes where possible.