feat: Support Adwaita for Steam

[mossymossmoss]

Feature description

Adwaita for Steam is a theme for Steam which brings it close to what standard Adwaita apps look like. It supports recoloring, and having this done when using Gradience to apply a theme would be useful

Additional Context

No response

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[tfuxu]

There is already a plugin request for it here: GradienceTeam/Plugins#1.
We might look into that soon, because color theme support was recently merged to Adwaita for Steam.
For now I'm closing this issue as duplicate, check the above issue for updates.