If you only want to change the appearance of certain words/sentences (e.g. highlighting), you could use the Custom Styles add-on.
If you want to change the styling of the editor itself, there’s the add-on Customize Editor Stylesheet, but it won’t allow you to edit everything because the field’s default styles are protected within a shadow-DOM. You can see this in action when you inspect fields with AnkiWebView Inspector.
About AnkiWebView Inspector
This add-on is really helpful for any kind of front-end tweaks or complex card templates.
I highly recommend you analyze the structure of the editor page with that inspector, then you’ll know which classes to style and which CSS-variables to use.
To fully control the look of elements inside Anki’s editor fields, you need to inject your CSS into the shadow-root with your own add-on:
(The Custom Styles add-on does the same thing, but you can only change the injected stylesheet through the add-on interface, which is a little bit limited.)