Themes are generally supported fine, as long they stay simple (for example, the default Twenty Xyzteen family of themes works without any issues). However, some themes might do a lot more than just simple theming, for example, certain "page builders" are popular and they are more like complex plugins, sometimes with custom database tables, etc., which means concerns for VersionPress compatibility.
This page lists supported, unsupported and partially supported themes.
As stated above, most simple themes are supported out of the box. That should be around 99% themes by our strict scientific measurements.
Headway page builder is currently incompatible with VersionPress because it uses external database tables that we don't track yet.