Theme #236
Description
Establish a theme.
Consider tamagui, nativewind.
An important part of this will be how the headers a displayed. The plant detail screen currently has a custom image header, while all other screens use a default navigation header. a) we should make them all pretty, and b) the mixed use is causing some visual glitches (eg when going from Todolist -> Chore Detail) so we want to make them all consistently custom.