Fix themes

[Thomas-Gallant]

Create official color palette for Sudokuru light and dark mode.
Remove duplication in the theme declaration (Right now theme colors are declared twice)
Make sure all components are using theme colors and are consistent (No hardcoded colors)
Make sure light mode and dark mode is readable everywhere

Have names for the themes instead of on / off switch toggle.

Stretch goal - add real dark theme and real light theme in addition to "Sudokuru" themes. For a total of 4 themes