Request: Add a "Greybird-Darker" theme

[okamsn]

Some themes, such as Arc, provide 3 variants:

1. A light version
2. A darker version, where the window decorations are dark but not the window content
3. A fully dark version

In Xfce 4.14, I used the "Greybird Dark" window decorations to do this, but this is inconsistent with programs using the GTK header bars. Please consider adding a version of Greybird where the newer GTK programs are wrapped by the dark window decorations, as seen in the images below.

What I was doing:

What programs look like with the GTK header bars: