Extend Theme via Interface

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Is your feature request related to a problem? Please describe.
With Nextcloud >25, theming was changed drastically, documentation is not present and based on my tests (and some comment in some nextcloud issue) I notice that actually there isn't any interface to extend theme capabilities.

We can create custom app and import/extend javascript or set app configuration (like color or other option as example in Breeze Theme or Pride theme) but if I want, for example, set a new Logo via theme and regenerate logo image, this is not possible becasue functions are not available.
Or, for example, if i want to change the foother section, like adding date or custom links and so on, via custom app actually is not possible (is still possible via /theme/<custom_theme>/defaults.php , that should be deprecated).

Describe the solution you'd like
Create a theme Interface or some extend solution, that can help us to extend theme functions or overwrite them

Describe alternatives you've considered
The alternative that we have are :

• create new app - force CSS for section like logo or background + usage of occ command to regerate logo and so on
• create new app - force gobal css variable

Additional context
Add any other context or screenshots about the feature request here.

Start by reviewing the existing theme path /theme/<custom_theme>/defaults.php and the customization approaches described in the issue, including app configuration and the occ command. Determine the intended extension points for theme capabilities such as logos and footer content, then define an interface and clear documentation for supported customizations.