Fix color palette changing after saving config in fullscreen

Summary:
If one opens the config with a keyboard shortcut in fullscreen mode, changes something,
and saves it, the color palette reverts to Normal (non-fullscreen), most noticeably changing
the background color.

This fix checks the fullscreen state when applying color palettes.

Test Plan: Change config while in fullscreen - colors should stay the same.

Reviewers: Gwenview, rkflx, muhlenpfordt

Reviewed By: Gwenview, rkflx, muhlenpfordt

Subscribers: muhlenpfordt

Differential Revision: https://phabricator.kde.org/D10501