This theme is just an extension of the default helix gruvbox theme that changes some small things to make it look more like the gruvbox community vim theme
To use the plugin you can create a file in .config/themes/{filename}.toml and paste the content in it.
My setup it's using stow so I clone the repo
git clone https://github.com/jeronimobarea/helix_gruvbox_community.git ~/.config/helix/themes/custom/helix_gruvbox_communityinside .config/themes/custom/ and the just run stow helix_gruvbox_community
which will create a symlink in the .config/themes folder.