Comment 4 for bug 1733707

Turns out I was over complicating things and didn't realize that Ambiance and Ambiant-MATE diverged a lot when they were altered to support GTK 3.20. Here's what should be done to fix this bug in both Ambiance and Radiance in MATE:

sudo apt install -y ubuntu-mate-themes ubuntu-mate-artwork light-themes ubuntu-artwork

sudo cp --target-directory=/usr/share/themes/Ambiance/gtk-3.20/apps/ /usr/share/themes/Ambiant-MATE/gtk-3.0/apps/mate-applications.css
sudo cp --target-directory=/usr/share/themes/Ambiance/gtk-3.20/apps/ /usr/share/themes/Ambiant-MATE/gtk-3.0/apps/other-applications.css

sudo tee -a /usr/share/themes/Ambiance/gtk-3.20/gtk-main.css <<EOF
@import url("apps/mate-applications.css");
@import url("apps/other-applications.css");
EOF

sudo cp --target-directory=/usr/share/themes/Radiance/gtk-3.20/apps/ /usr/share/themes/Radiant-MATE/gtk-3.0/apps/mate-applications.css
sudo cp --target-directory=/usr/share/themes/Radiance/gtk-3.20/apps/ /usr/share/themes/Radiant-MATE/gtk-3.0/apps/other-applications.css

sudo tee -a /usr/share/themes/Radiance/gtk-3.20/gtk-main.css <<EOF
@import url("apps/mate-applications.css");
@import url("apps/other-applications.css");
EOF

I tested this in 18.04, but I think this should work in 16.10 and later.

I'm sure that there are a few more improvements that could be done for MATE compatibility in general, but at least this fixes the mate-panel issue, and makes MATE apps look better in Unity as well.