Section Card: Support theme-based styling and optional background image

[rgr101]

Describe the problem you have

Section Cards are a useful way to structure dashboards, especially for mobile use. However, they currently use hardcoded styles (font sizes, spacing) and offer limited options for visual customization.

Describe the solution you’d like

Extend Section Cards in two ways:
1. Use theme-based styling:
Replace hardcoded values (e.g. font sizes, spacing, text colors) with standard theme variables like --primary-text-color, --paper-font-*, etc.
This improves accessibility and mobile readability, and allows consistent theme styling across dashboards.
2. Add optional background image support:
Introduce an opt-in parameter (e.g. background_image) that works similar to the one in Area Cards. This would allow a visual background to help visually differentiate sections, particularly in large or mobile dashboards.

Both proposals build on existing Home Assistant frontend features (themes, background handling) and do not introduce breaking changes. They would simply enhance flexibility and allow users to fine-tune the appearance of Section Cards more effectively.