Update theme styling

This is a follow-up to issue #2.

#### Summary of changes
- added basic non-editor and image viewer styling
- added overwrite cursor styling
- added matching tag styling
- changed selection color
- changed active line colors
- changed cm-builtin color (helps distinguish CSS ids from classes)
- removed gutter border
- window focus improvements

I've only tested these changes with the current master (0.43) on Windows. Feel free to merge as much or little as you want.