Use greyscale composite, the files are smaller.

themes/default/bottom-active.png

themes/default/bottom-inactive.png

themes/default/bottom-left-active.png

themes/default/bottom-left-inactive.png

themes/default/bottom-right-active.png

themes/default/bottom-right-inactive.png

themes/default/close-active.png

themes/default/close-inactive.png

themes/default/close-pressed.png

themes/default/hide-active.png

themes/default/hide-inactive.png

themes/default/hide-pressed.png

themes/default/left-active.png

themes/default/left-inactive.png

themes/default/maximize-active.png

themes/default/maximize-inactive.png

themes/default/maximize-pressed.png

themes/default/maximize-toggled-active.png

themes/default/maximize-toggled-inactive.png

themes/default/maximize-toggled-pressed.png