GTK3 CSD theme improvements

I was frustrated at the inability to re-size gtk3 windows with CSD as the hit-box for re-sizing was a 1x1 button, so I made some improvements to the gtk3 part of my workbench theme.

Now the CSD window borders are more consistent with xfce4wm!

Normal window.

The main trick was finding out which @#$@#$ classes to use for the css selectors. Almost every search sends you to out of date information that references '.window-frame', which is wrong. The GtkWindow documentation actually has the correct names.

And using the :backdrop pseudo-class is required for inactive windows.

Inactive window.

I also used padding and border colours so you get the minor 3D effect so they also match the xfce4wm theme.

Get the theme from code.zedzone.au, details are on the project page.