Basic setup (Colors, Fonts,...) where?

I wonder where I can set the basic bootstrap colours and fonts I want to use all over the project.
Do I have to make a theme for that? and if yes: how?
Any help very appreciated.

If you haven’t already, start by reading the program Docs top to bottom, and watch the videos on the Bootstrap Studio YouTube channel. Some of them are quite old, and there will be things in the older videos that no longer look like the current version of the software, but the basic principles in how the program works are more or less the same.

Also see my answer to your question here.