Add Screwdriver theme by Mister aiR

- Not include screenshot yet (wait
  https://github.com/marienfressinaud/FreshRSS/issues/571)
- Need maybe more tests?

See https://github.com/marienfressinaud/FreshRSS/issues/557
Merge branch 'dev' of github.com:marienfressinaud/FreshRSS into dev