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After restarting Mate, you will see that windows have wider borders,
so that's an improvement at least.
But they can be made even wider if desired.
As root, edit usr/share/themes/Spidey/metacity-1/metacity-theme-1.xml.
You will see these lines near the beginning:
<frame_geometry name="normal" rounded_top_left="true" rounded_top_right="true">
<distance name="left_width" value="3"/>
<distance name="right_width" value="3"/>
<distance name="bottom_height" value="3"/>
Changing '3' to a higher number, most important for right_width and
bottom_height, will widen those borders.