feat: Applying "Legacy Application" themes via Gradience

[orowith2os]

Feature description

Tweaks has a feature that allows the user to set "Legacy Application" themes; this is currently used so Gradience can theme GTK3 applications, by setting adw-gtk3. Gradience could set this option itself when applicable, so Tweaks is no longer required for people to set the GTK3/"Legacy Applications" theme.

Additional Context

It needs to be investigated to ensure that this will affect pure GTK4 (no Libadwaita).

We're looking into migrating this setting to Gradience to minimize the amount of features that Tweaks and/or a future Tweaks alternative needs to handle - Gradience, being a theming app for GNOME, seems like the most appropriate place to handle this.

https://gitlab.com/OroWith2Os/toggle

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[orowith2os]

The Flathub issue is no longer a blocker for this.

[0xMRTT]

@orowith2os what about merging the entire theme section of toogle into gradience ?

[0xMRTT]

Fixed!!!!

[0xMRTT]

@daudix-UFO can you try ?

[orowith2os]

@0xMRTT that could work, but I need to look at Tweaks for what's necessary to get this going first. And I'm not sure if handling non-GTK theming is in scope for Gradience yet.

[daudix]

@daudix-UFO can you try ?

Tested, works as expected