Install GTK theme
GTK conda package comes limited themes.
Add one in the sources for now and ensure it's picked when running GUIs.
One way to do it is to set it up in the launcher scripts. For exemple:
export GTK2_RC_FILES=<path-to-install>/themes/Yaru-dark/gtk-2.0/gtkrc # OK
Note: Windows install keeps its old gtk dependency for now. Check the themes inside and how they are installed ?