Make fullscreen palettes use system color scheme accent

Summary:
The Fullscreen palettes contain the default Breeze accent color (blue). This patch
replaces the accent color in the fullscreen palettes with the one from the normal
palette (which does use the system color scheme), adjusted to not be too bright.

Before:

After:

Test Plan:
Change color scheme to something other than Breeze (like Honeycomb), open Browse view
in fullscreen. Colors should match the chosen system theme (e.g. yellow for Honeycomb).

Screenshots using Honecomb.

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Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D10649