Checkbox icons in installer are illegible

This MR changes the background color to white, like the example image in the OP. If there are more specific designs to be implemented here let me know and I can try and update it.

On Firefox, I have it black with a white checkmark (without using the MR).

I tested this out in a couple browsers (Safari, Chrome, FF) and it was fine. So I think I must have been seeing a caching issue. False alarm. Thanks @poker10 for double checking.