TemplateResponse: Enforced theme not respected for `renderAs = BASE`

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Describe the bug
I have in config.php 'enforce_theme' => 'light', embedded forms are shown in dark mode (follows system settings)
If looks bad when embedding a dark form inside a light themed website.

To Reproduce
Steps to reproduce the behavior:

0. config.php 'enforce_theme' => 'light'
1. Create a form
2. Embed the iframe into another website
3. Set OS settings to dark mode
4. Check embedded form

Expected behavior
I would like the embedded forms to respect the config.php settings.

Nextcloud (please complete the following information):

• Nextcloud-Version: 31.0.5
• Forms-Version: 5.1.0

Desktop (please complete the following information):

• OS: macos
• Browser: Chrome
• Version: 136.0.7103.114

Smartphone (please complete the following information):

• Device: Pixel 9
• OS: Android 15
• Browser Chrome
• Version latest

Start by tracing TemplateResponse handling for renderAs = BASE and how the enforce_theme setting from config.php is applied to embedded forms. Reproduce the issue with the documented steps while the OS uses dark mode. Done means an embedded form follows the configured light theme rather than the system preference.