In my case, Godot editor always tries to change Color in editor/theme.tres, which makes the submodule dirty.
I searched for a solution but couldn't find a good solution.
If the repository owner has changed the interface/inspector/default_float_step setting, it may be a good idea to restore it to the default and continue development.
Or maybe changing the repository to include .res instead of .tres would solve the problem, but I'm not sure.
I reported it, but it's not a serious issue so you can probably ignore it.