I created an own theme for SXA incl. Gridding, the problem is, that SXA is still calling bootstrap.css, which overrides all the styles that I made in my own theme. Is there is a possibility to disable the Sitecore calls to bootstrap?
Bootstrap will be your grid system (it's the default). If you want to remove all references to bootstrap, I think you need to select another grid system (currenly you can select Bootstrap, Foundation or Grid960) or create your own.
Info on the grid systems: https://doc.sitecore.net/sitecore_experience_accelerator/building_the_layout/structure/the_grid_settings
How to create your own: Create new Grid in Sitecore Experience Accelerator