Themes do not update properly

When I select a different community theme, it does not update properly. My headings will not follow the ‘style’ of the new theme, some components on the screen change and some don’t. Not sure if there are some persistent files that I need to thrash to have it work again ? (I could change these themes in the past).

I have not worked with CCS snippets.

More than happy to provide logs if someone could point me there.

Have you re-installed the themes you’re testing? Do you have css snippets activated that could be colliding with your theme?