[UI] Automatic theme switching

Following up on this a quarter later, thank you very much for getting this sorted in the latest versions of 1Password for Linux! Matching the system theme mostly works like a charm.*

* There is a caveat that 1Password will not set the proper initial color scheme if the system scheme has changed from light to dark in the time period between the application quitting and starting up.

1. Set system scheme to light variant.
2. Launch 1Password and ensure its presenting in light mode.
3. Reboot system or completely quit 1Password.
4. Change system scheme to light dark variant.
5. Launch 1Password.

In this scenario, the title bar will correctly have the dark theme, but the actual application surface will be rendering with the light scheme. Switching the system state to light then back to dark brings 1Password back into the correct state.

1Password: 8.10.24
OS Version: Fedora 39