Board Thread:Support Requests - Designing Your Wiki/@comment-37993906-20190104001543/@comment-27345308-20190104010133

That is what happens when your header color reaches over a certain amount of "lightness". It happens because on a wiki with a white header, white icons and text would look bad. You might have to experiment with the colors a bit to see which ones aren't considered light enough to make a switch of icon colors.