gtkconfig: set `color-scheme` when current color scheme changes
This makes color scheme consistent between Plasma and GNOME. That is to say, when a user logs out from Plasma and logs into GNOME, the color preference will be kept.
Closes https://invent.kde.org/plasma/kde-gtk-config/-/work_items/13609
| Light | Dark |
|---|---|
 |
 |
Edited by Fushan Wen