More users means more exposure to problem points which may have previously flew under the radar. (Same reason why not all bugs are exposed during a beta program.)
As has been mentioned before, this is not merely a cosmetic preference, but an accessibility issue. Check out item #s 23-25 of the top Google result for “software accessibility”: https://www.justice.gov/crt/software-accessibility-checklist
|Color|23. Does the software ensure that color-coding is never used as the only means of conveying information or indicating an action?
|Color|24. Does the application support user-defined color settings system-wide?|
|Color|25. Is highlighting also viewable with inverted colors?|