Soft and Hard variants of TheGreatMcPain/gruvbox-material-gtk.
1 - Ensure that you have the .themes directory: mkdir ~/.themes
2 - Clone the themes: git clone git@github.com:gustavothecoder/gruvbox-material-gtk.git
3 - Enter in the themes folder: cd gruvbox-material-gtk
4 - Copy the desired themes to the .themes folder: cp -rt ~/.themes/ ./Gruvbox-Material-Soft/ ./Gruvbox-Material-Hard/
5 - That's it! Now just select the theme.