Themes for The GNU Image Manipulation Program 3.0
How to install:
Launch GIMP and go to Edit > Preferences > Folders > Themes and see where it looks for the themes, put the custom theme in that folder.
Alternatively, click the button "Add a new folder" and give it a new path to search for the custom theme(s). (needs restart)
DarkMetallic
Download: here
Blender
Download through download-directory.github.io: here
Fantasy (with swap-out textures and secret light theme variant)
Download through download-directory.github.io: here
Graycast
Download through download-directory.github.io: here