useTheme: add support for `system` theme

[McManning]

Should be able to set a non-explicit theme that falls back to user's prefers-color-scheme.

Related to #52

[McManning]

Bumping down priority on this. After moving to MUI for 5.0-beta, I never reimplemented the dark theme. I'm going to hold off on that for now since it's not a huge priority for us (but a really nice to have)