HyperflatGraphite
- Create a new folder e.g ‘my-gimp-themes’ anywhere on your computer where you're unlikely to accidentally delete or move it in future, and then place the unzipped HyperflatGraphite folder inside it.
- Then, in GIMP, go to Edit > Preferences > Folders > Themes, click the ‘Add a new folder’ button on the left and then the ‘Open a file selector…’ button on the right and select the my-gimp-themes folder you just created.
- Click the OK button.
- Restart GIMP.
- Go to Edit > Preferences > Interface > Theme and select HyperflatGraphite
Read more in the GIMP Manual