Improve selection and hover colors in the thumbnail bars

Summary:
The visual feedback when hovering, especially in the fullscreen thumbnail bar,
was too weak. I've fixed this by improving the hover background color, and
applying a stylesheet to the fullscreen thumbnail bar.

Before:

After:

Test Plan:
Thumbnail bar colors in both View mode and fullscreen View should provide
good visual feedback (hover and select).

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: rkflx, ngraham

Differential Revision: https://phabricator.kde.org/D10564