First of all, you should have BetterDiscord up and running. You can install that here:
After this, you can install the theme:
Just click "Download Raw File", and choose the desired location.
Now, put the recently installed Gruvdisc.theme.css
in your .config/BetterDiscord/themes
folder. If you are having trouble finding the folder, head over to the "Themes" section in discord with BetterDiscord installed and click on "Open Themes Folder". All you have to do is drop your file there.
That's it! The only step left is opening Discord and enabling the theme. (It's also has a support for light mode)