popupMenu: Restyle the slider to use a color for the 'active' area

popupMenu: Restyle the slider to use a color for the 'active' area

In the mockups the slider does not have an uniform color but uses
one color (shade of blue) to indicate the current value and one
to indicate "the rest" (shade of grey).

So adjust the slider to look like that to be closer to the look
in the mockups and thus to the design.

https://bugzilla.gnome.org/show_bug.cgi?id=644600