You used to be able to "suggest" colors to use, but allow websites to override them. For example, I could have a bright green background set as a preference, but allow the website to override that preference. This allows a developer to determine if they have properly defined all their colors more easily. I've attached an image of how it used to work: if the site defined colors, Firefox would use them, but if they did not, you would get the user preference (in the case, green and red for bg and fg colors). This will clue the developer into realizing that they are assuming white and black as colors.
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