Foldit's main window has a light background by default. In the "Advanced GUI", the view option "Show dark background", switches to a dark background.
The light background is not quite white, and the dark background is not quite black.
Screenshots made with a light background have a slight yellow cast, particularly when viewed on this wiki.
The background color is controlled by two settings in the file theme.txt, found in the Foldit installation directory. The options are "gui/color/bg", the light color, and "gui/color/bg_dark", the dark color. The default settings are:
"gui/color/bg" : "255 255 234" "gui/color/bg_dark" : "12 12 12"
These are standard RGB (red, green, blue) values.
Change the settings as follows to get true white and true black backgrounds.
"gui/color/bg" : "255 255 255" "gui/color/bg_dark" : "0 0 0"
These settings may have some side effects. For example, if "AAColor" coloring is selected, glycine is shown with a blue-grey color. If you hover over glycine with AAColor, it changes to pure white. This causes glycine to disappear against a pure white background. There may be other minor changes noticeable if you modify the background colors.