Hello,
Now that's a complicated, potentially conflictual topic, nearly as much as the FreeCAD logo...
Some history, if anyone cares about it. The FreeCAD default background colour dates back from its early days, nearly 20 years ago. Juergen Riegel (FreeCAD's founder) based it on CATIA's, which he considered the pinnacle of CAD software. Much of early FreeCAD features are inspired by CATIA.
I've been a 3D CAD user for 24 years (starting on AutoCAD/Mechanical Desktop with a pure black background). Contrary to what's been suggested here, background colours do change over the years, at least in the commercial CAD world - that is, for other software than CATIA!
The trend in the industry has been leaning toward light backgrounds. Even pure white in some cases. You can search for images of Solidworks, Inventor, Fusion 360, Solid Edge, Onshape, PTC Creo Parametric or Elements.
Solid Edge has a black and grey gradient I find unsuited to the other colours in the program; in the sketch mode, dimensional constraints are dark red, and when they are on a darker part of the screen, they're not very readable.
I use a straight white background, I don't find it tiring for the eyes. But I do use CAD at work during the day, rarely in low lighting conditions.
I know that this is mostly an aesthetic issue, and that a background colour can be changed by the end user, but the current one feels extremely outdated. Updating it for the 1.0 release is a good idea. Personally I would suggest a light grey gradient, something relatively neutral. But as it's been mentioned, no matter the chosen colour scheme, it will require adapting the Sketcher colours as well.
Anyway, that's my two cents...
Cheers
P.S. Solid Edge does offer multiple colour themes such as "Classic", "Solid Edge 2000" for the old-timers...