A few possible answers.
Sometimes a page doesn't load properly, and one would have to refresh. Though, this behaviour is more obvious on looking partially wrecked (things not in the right places, long strings of text), and doesn't present itself as a color change.
Sometimes we force a style (color and/or layout theme) upon a forum. For example, the Festivus forum currently has a dark theme we're trying out, though it usually has a more festive holiday theme when we open it for Christmas-NewYears. We also turn the whole site to a black-and-white dark theme in February to recognize those we've lost over the years, but that is site wide. We don't currently force a style anywhere else, though there are other styles available.
If you go to the bottom left of any page there is a style chooser (admittedly, it's damn near invisible in our standard "BL Style")
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Clicking on that brings up a window of styles you can choose from which is currently pretty slim, but is available to you with a "BL Dark" which you may have slipped into and out of - you can choose this as your style for all forums (except where we force one) if you wish.
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We do hope to fine tune the darker one in Festivus and make that a regular option as well in coming months and perhaps others along the way. Right now, "BL Style" (our standard light blue) and "BL Dark" (a b&w version) are all we offer.
Does this help?