x11-themes/canta-gtk-themes: Fix xfwm4 theme

Fix bug where the xfwm4 theme remains canta-dark no matter which
wm theme you choose from canta. While here tweak pkg-descr accordingly
to the other theme ports that i maintain.

PR: 260506
MFH: 2023Q2
(cherry picked from commit fc8585d9420f0b1b0ebf922810cccbaa81400844)