Feature Request: add optional light mode / dark mode control

[bouts-gorse]

Is your feature request related to a problem? Please describe.

• The wallet UI automatically follows system settings.
• It would be nice to control light or dark mode manually.

Describe the solution you'd like

• Add an option to force light mode or dark mode.
• Make it configurable during wallet setup.

Additional context

Why it would help

• Gives developers more UI control.
• Makes the wallet feel more integrated in apps.
• Helps with custom theming.