Rockbox

If you have a progress bar 100px wide and a background image for the progress bar 100px wide you can place them on the theme with this code:
%pb(0,0,100,15,pb.bmp,backdrop,bd.bmp)

your progress bar will line up perfectly with your theme.
if you then change the above code to:
%pb(0,0,100,15,pb.bmp,backdrop,bd.bmp,slider,slider.bmp)

the progressbar and backdrop are shrunk and neither line up with your theme correctly.
in addition the slider apears to hover over the ends of the progressbar instead of lining up with the ends of the bar.

there is no simple way to line up the progress bar with the theme and keep the slider.

example:
if you are making a theme for an ipod Video (320px wide) there is no way you can make the progress bar 320px wide. if you have a slider that is 20px wide your progress bar appears to have 10 px padding on either side of the screen. The progress bar and background dont fill the whole width of the screen. There is no way to get around this.