There is really no limit to how you can style them. If you don't like the default focus ring, you can remove that, but make sure to put some kind of styling back.
Here I've used a header element for each expandable section, which has a focus state that mimics other interactive elements on the page.
The only browser that doesn't support this are the Microsoft ones (and Opera Mini which makes sense—it doesn't really do interactive).
This browser support data is from Caniuse, which has more detail. A number indicates that browser supports the feature at that version and up.
Mobile / Tablet
But even then, it's just like all the sections are opened, so it's not a huge deal:
Wanna style that default triangle? Strangely enough, the standard way to do that is through the list-style properties. But Blink-based browsers haven't caught up to that yet, so they have a proprietary way to do it. They can be combined though. Here's an example of replacing it with an image: