[ENHANCEMENT] Background of puzzle board panel in dark mode should not be light gray

[charlestian23]

Describe the current behavior of what you're trying to improve. If your enhancement request related to a problem, please also describe the problem.

Currently, when setting LEGUP to dark mode, whenever you open a puzzle, the puzzle board panel is light gray.

Describe the improvement you'd like

The panel background should be dark gray like it is in the rules panel background and the proof tree panel background.

Describe alternatives you've considered

No response

Additional Context

No response