UI enhancement: Use solid card background to improve readability across themes

The main content card currently uses a transparent background. While visually appealing, this can reduce text readability on light theme

Using a solid (non-transparent) background would improve clarity, contrast, and overall UX consistency.

This enhancement is related to the light/dark theme consistency discussed in [#64](http://github.com/Alchemyst-ai/awesome-saas/issues/64)

### Expected behavior

- Cards use a solid background color instead of transparency
- Improved text contrast and readability in both light and dark modes