Improve background color code for URL Navigator and adjacent fullscreen toolbars

Summary:
Similar to {D10781}, 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:
{F5731730}

Fullscreen left edge after:
{F5731731}

Fullscreen right edge before:
{F5731732}

Fullscreen right edge after:
{F5731733}

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

In {nav Fullscreen} {nav 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