Improve Oxygen Theme

The latest Oxygen theme had several issues regarding how themes work. Notably:

• Removes button shadows -- There doesn't seem to be a workaround for non-active shadows, so it's removed since you can't see them anyway. This temporarily fixes the "weird black shadow" on inactive buttons until shadows can disable themselves for inactive buttons
• Fixes tab bars -- Adds east, south, and west variations
• Fixes active button size -- Not sure why this one works, as the SVG file for active buttons actually has a smaller set size, but this sort of fixes the buttons expanding
  • (I think it might have something to do with the inset shadow? Could be a bug)

It still has some visually that look odd, and there really isn't an Oxygen guideline these days, so I may do some touch ups in another PR.