Remove current Dark Mode implementation

[mariobehling]

Dark mode design usually requires a dedicated, well-thought-through approach. The current implementation only adjusts some background colors to black, which causes several problems:

• It clashes with older screens that were not designed for dark mode
• Red and black combinations on certain pages create visual inconsistencies
• Overall UI stability and consistency are negatively affected

To avoid confusing or inconsistent UI behaviour, please remove the current dark mode implementation entirely.

We can revisit and design a proper dark mode once the app is more stable and the remaining UI components have been updated.

Tasks

1. Remove the existing dark mode theme and related code
2. Ensure the app consistently uses the standard light theme
3. Clean up any leftover styling attributes that assume dark mode
4. Verify that all screens display correctly under the light theme