New GUI doesn't respect theme

New GUI doesn't respect theme, when using Yaru Dark

How, and what happened?
1. set yaru dark theme on Ubuntu
2. use the new GUI for Multipass

**Expected behavior**
It should have respected the Yaru Dark Theme, however it doesn't and the main part of the window has a white background.

**Logs**
Can't see anything on the logs by running: "journalctl --unit 'snap.multipass*'" as sugest on the documentation, so if there's another log that is relevant please ask me for it

**Additional info**
- OS: Ubuntu 24.04
- I'm running multipass 1.14.0

**Additional context**
This is a screenshot of the app as rendered to me:

Start by reproducing the issue on Ubuntu 24.04 with Yaru Dark and the new Multipass GUI, using the reported steps; the issue names no source files or tests. Trace the GUI's theme handling and verify that the main window uses the dark theme instead of a white background.

Written by the indexing model from the issue text.