its controlled by constants.css
line - @const TOOLBAR_BG: #f5f5f5; controls the toolbar colour - not checked status bar but that probably uses the same constant - will let you check
if you lift the variable - set_alpha(@c_tab_active_bg_temp, 55%) - from skin_c_light.css and add that then it alters the appearance
so line becomes - @const TOOLBAR_BG:set_alpha(@c_tab_active_bg_temp, 55%) ;
to adjust the transparency level play with the 55% figure
simple