Widgets dont receive a theme if the settings are set to "system"

### Steps to reproduce

1. Use the system theme in element-web
2. Have a widget defining `$org.matrix.msc2873.client_theme` in their URL

### Outcome

#### What did you expect?

It translates "system" to either "dark" or "light"

#### What happened instead?

It omits the theme entirely and causes a default to light therefor

### Operating system

Fedora Silverblue 40

### Browser information

Firefox 129.0 (64-Bit)

### URL for webapp

develop.element.io

### Application version

Version von Element: 43fa2e533f79-react-9f5b39b9d951-js-d6080398dbf5 Krypto-Version: Rust SDK 0.7.1 (c8c9d15), Vodozemac 0.6.0

### Homeserver

matrix.midnightthoughts.space

### Will you send logs?

No