Making a demo in an empty sky is pretty easy, here's all you'll need to do:
Open a demofile with notepad (or any other text editor). In the first few lines, you'll see something like this:
1 0 Version 2
0 0 Map maps/City_Zones/City_01_03/City_01_03.txt
0 0 Time 13.540822
All you have to do to make an empty sky is take away the part where it calls up the map in line 2, so it should look like this:
1 0 Version 2
0 0 Map
0 0 Time 13.540822
Also note that line 3 is the time of day, so if you want a daytime sky, put a number there to reflect that. As you can see it's recorded in military time, so an entry of 9 would be 9AM, and 21 would be 9PM.
The demo doctor tab in my demo launcher will remove the map for you if you want. It won't set the time of day, though.
You can't get a solid white, as these guys have suggested you can use the empty sky OR... if solid BLACK will do, there is a way to get that.
Load the characters in the sewer map and set the coordinates beyond the walls of the sewer tunnels, volia blackness all around you.
w00t Radio
Is there a way to edit a demo so that it takes place on a unmarked white background? Or a background that's just distant sky or abstract "wallpaper"?