Enabling Dark Mode
Press "File" in the top left corner and and select "Options" (1), now select the drop down for "Select theme" (2)
Choose "Normal" for the bright design or choose "Dark" (3) for the darker design.
Remember to save your project before changing the looks of Project Designer, because the software will restart!
The dark mode can be more restfull for the eyes if you are working in front of a large monitor over longer periods of time.