Pass Environment Variables to "Custom Command" #51

Hi,

I am new to Workspaces but I like it. Thank you very much!
When I try to run a custom command like:

 /usr/bin/evince /home/user/Documents/file.pdf

I starts my pdf viewer just fine. However, when I want to pass an environment variable on like this:

GTK_THEME=Adwaita:dark  /usr/bin/evince /home/user/Documents/file.pdf

Evince does not start any more.

It there a (correct) way to pass environment variables to the custom command?

Thank you very much!