Current Solution (2019) for Ubuntu-MATE 16.x, 18.x**

(possibly more, pls add to comments)

Yes, those borders are a pain, bordering on –pun intended– masochism...

1 create copy of a theme

Well, ClearLooks is (for some reason) not really a "full theme" to pick on the Theme tab, but rather a "Border Theme" under „customize..“ on the Window Border Tab:

Well, never mind, let's create a copy (use rsync or cp as you please):

(If you are fond of another "border theme" then duplicate that one accordingly. Look it up under customize theme-> Window Border what you are currently using.)

sudo rsync -vr /usr/share/themes/ClearlooksRe/* /usr/share/themes/ClearlooksReBIGGER

2 change border width in the xml File

sudo pluma /usr/share/themes/ClearlooksReBIGGER/metacity-1/metacity-theme-1.xml

There are actually two places:

• under <frame_geometry name="normal"...
• under <frame_geometry name="border"...

For left, right and bottom I went for a rather generous 12px:

<distance name="left_width" value="12"/>    <!-- FRANK: was: 4 --><distance name="right_width" value="12"/><distance name="bottom_height" value="12"/>

3 pick the new Window Border theme and there you are

Some background on metacity themes, if you care.