Widget background color set by theme

#### Describe the bug

This is same bug as:
https://github.com/vector-im/element-web/issues/16919

Widgets are web pages in iframe. Web pages have white background by default.

Element web's widget viewer uses background color from theme. This causes
many web pages to have black text on dark background with dark themes.

#### To Reproduce

Steps to reproduce the behavior:

1. Create a widget, for example https://tkstats.r-f.fi/
2. Set dark theme
3. Observer page being unreadable

#### Expected behavior

Readable page with white background

#### Smartphone (please complete the following information):
- Device: Cosmo Communicator
- OS: Android 7.0 (?)

#### Additional context
- App version and store latest - gplay
- Homeserver: hacklab.fi