The theme files will be under /usr/share/themes/
. You'll most likely need to edit the CSS files in the gtk-3.0 subdirectory.
Use gcolor2
to find out the current colour of the toolbar - say it is #eeefff
. Now figure out what you want the colour to be - say #ababab
.
now you update the files - the easiest way is to just replace all occurances of the old colour with the new:
find /usr/share/themes/Cupertino -type f -exec sed -i 's/#eeefff/#ababab/g' {} \;
of course this will replace the old colour with the new everywhere - which may not be what you want. If you want to change the colour of the toolbar only you can figure out which css class specifies the toolbar (experimentation and Google will help here) and change just that.