gruvbox-material-gtk is deprecated

https://github.com/TheGreatMcPain/gruvbox-material-gtk/commit/bb306ae972273cbfcbf78f8b772662e8b0678d82

One option is to migrate to https://github.com/Fausto-Korpsvart/Gruvbox-GTK-Theme, but not sold on that theme to be honest