cs_themes: Set value of xsettings on gtk theme selected

[ArqamQazi]

Problem

Related to #12994, see my comment.

When the GTK theme is changed from advanced option it is not reflected on apps like firefox/libadwaita/flatpack apps.
I have to manually set prefer dark or light in the setting.

theme-not-working-properly.webm

I modified _on_gtk_theme_selected to set the xsettings value, the value of org.x.apps.portal color-scheme is decided and set based on theme's name.
After applying changes:

cinnamon-2026-05-03T151933+0530.webm

[anaximeno]

I'm not sure if this is the best way to implement this. Because won't work well if a theme is a dark theme but just happens to not have "Dark" in the name (on the theme download section you can find some). Or as I see it goes to 2 in case neither darker or dark are found in the name, which I assume is prefer-light? In the case of a dark theme which just happens to not have "Dark" in the name it would switch to the prefer-light instead if that's the case.

[mtwebster]

We set this in the 'simplified' view for the user already.

In 'advanced' mode where you can set individual toolkit themes, there's already a way to set this in the 'Settings' tab ("Dark mode"). There doesn't need to be any additional magic here, just use the controls already available, or use the 'Simplified' view for themes that support that.