[feature request] Expand Theme Support to Ozone

One of the best features of XMB is its ability to switch between "themes" which are made up of custom icons, wallpaper and fonts. It would be great to see that capability be added to Ozone. While Ozone has two hard-coded color themes, the customization options deserve be greatly expanded.

Since Ozone took over as the default menu driver, it means fewer end users have direct access to customize their experience. While the monochrome icons are nice and serve their purpose, end users should be able to customize their experience. While some people still use the XMB, I'm sure there are plenty of people who'd like to be able to tailor Ozone to their own preferences.

Over on retroarch-assets, this feature has been discussed, but never implemented. Most of the contributors on this repo are designers and artists, so we need some help from developers getting this feature added to the main repo.

Start by reviewing the Ozone menu driver and the related discussion in retroarch-assets issue 341. Clarify how themes should be represented and applied before changing the implementation. Done means Ozone supports the requested customization of icons, wallpaper, fonts, and color themes.