Improve background color code for URL Navigator and adjacent fullscreen toolbars

Summary:
Similar to D10781: Replace manual palette adjustment with background role (fullscreen info label), modifying background role to get a dark background instead of
manually adjusting palette is a preferred solution. This way the palette is still
inherited so does not need to be continually adjusted when the overall palette
changes, like when switching to/from fullscreen.

This also makes the entire fullscreen toolbar (including URL nav) have a consistent background color

Fullscreen left edge before:

Fullscreen left edge after:

Fullscreen right edge before:

Fullscreen right edge after:

Test Plan:
In Browse, the URL Navigator should have a darker background color than the rest of the UI.

In Fullscreen Browse, the entire toolbar should have a consistent, dark, background color.

Reviewers: Gwenview, rkflx, ngraham

Reviewed By: Gwenview, rkflx, ngraham

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D10880