old topic but I have a similar issue, started noticing when installing (officials) theme components (Hamburger Theme Selector and discourse-category-banners and Dark-Light Toggle), I’m self hosted running 2.9.0.beta14.
I had changed the sub groups to 3 in this way: https://meta.discourse.org/t/sub-sub-categories/139309/7?u=sjefromijn and changed back to 2, but unsure if related.
I chose the default theme with ‘shades of blue’ color scheme during installation/wizard, but it’s stuck there and none of the theme components work. Where to look or change anything?