Rework theming to use more defaults

[CrazyStevenz]

Right now, a lot of dark theme color settings are set inside the components themselves, ovewriting defaults. This happens due to two mistakes made during the original theming process:

1. The navbar uses the primary.dark color instead of a dedicated accent color

2. The primary colors for dark theme are the accent colors instead of white

[CrazyStevenz]

Material UI v5 is reworking theming so we're going to wait for them to complete it before making any changes.