Do you mean the background in the image part of the window itself? Or the uniform color around the buttons?

In the image sub-window itself, you can do 2 somewhat different things. The "Swap" button (near the upper right corner) will invert the color scale, which will make the background white.

But if you just want the background to be white, with the other colors NOT inverted, you can do this:
1) Right click in the grayscale bar just to the right of the image - this will popup a menu
2) Click on "Choose Zero Color", and pick the color you want to fill in for voxels that are 0.

Yet another way to do something similar is to press (and release quickly) the "up" arrow button next to the "r" at the right of the image viewer. This will rotate the grayscale bar one notch up, which will put the top color (white) at the bottom of the scale.

Both of these latter 2 methods will also fill any any background voxels INSIDE the image with the white color, rather than just alter the voxels exterior to the central image. If you don't want this effect, then you have to do something yet different. I won't describe that just yet.