Theme builder

[mrosvik]

### Tasks for Beta version (29. mai)
- [x] Bachelor's thesis with interviews and user testing of prototype
- [ ] https://github.com/digdir/designsystemet/issues/1036
- [ ] #1377
- [ ] https://github.com/digdir/designsystemet/issues/1843
- [ ] https://github.com/digdir/designsystemet/issues/2044
- [ ] https://github.com/digdir/designsystemet/issues/2045
- [ ] The switch button checkmark has wrong color

### Tasks for later versions
- [ ] Make use of Brreg Contrast checker in the theme builder https://contrast.brreg.no/