Allow custom styling for the public embedded calendar

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

The built-in iframe embed doesn't allow any visual customization. Colors, fonts, and layout are imposed by Nextcloud, making it impossible to integrate the calendar seamlessly into a website with its own visual identity.

Describe the solution you'd like

Basic theming options for the public embed, for example:

CSS custom properties overridable via URL parameters or a configuration UI
A choice of accent color / color scheme
A minimal/compact layout option suited for embedding in third-party websites (Wix, WordPress, etc.)

Describe alternatives you've considered

I bypassed the iframe entirely by fetching the public ICS URL directly, parsing it server-side in PHP, and rendering a fully custom HTML widget styled to match our association's visual identity. Here is the result:
https://cafeassocugnaux.wixsite.com/huit/evenements
This works, but it requires significant technical effort that most non-technical users cannot do. Happy to share the full source code as a reference or starting point.

Additional context

Related: #3416, #2938

Start with the built-in public iframe embed and review related issues #3416 and #2938. Clarify the scope of theming before changing code: URL or configuration-based CSS customization, accent or color-scheme selection, and a compact layout. Done should provide an agreed, usable customization path for third-party embeds without requiring direct ICS rendering.