If you want to try some hacking:
Install GIMP beta from Flathub-beta (see my repo list which contains it as an example
And try to add the theme PhotoGimp
As Gimp 3.0 is nearly done, Photogimp likely needs an update as its made for GTK2.
GIMP 3 introduces some necessary features that are a base requirement for graphics design, nondestructive filters and color profiles (if you want to print)