I have really fallen in love with Google's "Material Design" when it comes to building UI for web/mobile/desktop apps. However, it was also the first time that I had encountered such a rich style guide for UI (I come from a web dev background). Are there any other similar style guides, but based on other design principals or aesthetics?
There are like a million of them... They're called Design Systems, and probably this link will help find the one you love:
Is this not obvious?
If Google has one... the other players should:
Some others are a bit more modest.
In fact, you can make a search using those terms: https://duckduckgo.com/?q=UI+guidelines
Do you look for a fully equipped framework, or just UI design kits? Probably Bootstrap Themes are worth looking into if you need a full framework with different theme options. UI Kits (just the design templates) can be found in abundance via a Google search for UI Kits or on commercial sites like UI8.