Theme module fixes

make sure psr4 themes work and change one core theme (SeaBreeze) as a demo. I think this may close #1949 but not 100% sure.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| Refs tickets  | #1949
| License       | MIT
| Doc PR        | -
yes. I left it to make sure the transition would work.