I am constantly finding applications that - often unconsiously - assume the end-user hasn't changed the system colours.

\r\n\r\n

Used to be, once upon a time, that people would specify a font color and never realize it might conflict with something the user had specified for a background, or vice-versa. These days the W3C CSS validator will throw a warning if you specify a foreground color without a background color, or a background without a foreground.