Correctly align tab and toolbar backgrounds with the frame background.
This fixes off-by-two/off-by-three errors in vertical positioning for themes
that tried to provide aligned tab/toolbar and frame assets. Tested with several
such themes, including both themes that did and did not provide a custom
background tab image.
TEST=Apply the "Dots" theme from the webstore and open a restored-mode window; the alignment of the background images in the frame, background tabs, and foreground tab should all match.