In recent years more programs have added a Dark Mode/Theme and I've found it to be much easier on the eyes for those of us who spend a lot of time staring at a user interface.
I've been using dark mode for Outlook, Visual Studio, etc., for years now and I've definitely noticed a difference.
After working in VS or something else with a dark theme background, switching over to the Workflow Designer or desktop client is a bit like flipping on the lights in the middle of the night and when I have to spend a lot of time in there working on something it would be nice to have a display option that's a bit easier on the eyes.
I'm sure it would take quite a bit of effort to go through and set up all the necessary styles for a dark theme option, but I for one would be extremely happy/grateful to have that option, especially in Workflow Designer.
EDIT: I see from the Related Post section that this has been mentioned before for various things like the client and Forms too.