Migrate as much as possible to LaTeX3

- [x] Preamble an option declarations
- [ ] Create new variants of `size1x.clo` and maybe `fleqn`/`leqno.clo`
- [x] Command/environment definitions
- [x] Color themes
- [x] Everything else
- [ ] Update all documentation
- [ ] Regression test at least all the macros and options defined