gui2/tprogress_bar: Use same background as listbox (children) widgets

Replaces the solid RGB background with the generic RGBA background
defined in GUI__BACKGROUND_COLOR_ENABLED.

data/gui/default/widget/progress_bar_default.cfg

@@ -41,7 +41,7 @@
 					h = "(height - 2)"
 					border_thickness = 1
 					border_color = "0, 0, 0, 255"
-					fill_color = "21, 22, 24, 255"
+					fill_color = {GUI__BACKGROUND_COLOR_ENABLED}
 				[/rectangle]
 
 				# 3D effect at upper side.