Dark Mode Improvements

We have basic support for Dark Mode, but some things could be improved: 1. Switch to a different set of SVGs that are optimized for Dark Mode, or at least optimized to be okay in light or dark 2. Toggle the material for the Gizmo--currently it's just a middle ground that looks okay in both 3. Toggle the material for solids that you make 4. Toggle the material for Faces so that you can more clearly see when a Face is being moused over 5. Toggle the material for Planes so that they are slightly lighter instead of slighter darker in Dark Mode