Since the option for changing background text is provided, we can assume people who use this option want to deviate from what their theme tells the panel to do. In this case, it sometimes becomes impossible to use this feature, as the text color still follows the theme. For example, the theme might give a light panel with black text, but the user might want to switch to dark, which would obviously have a white/light text color.
Since the option for the background color is provided, it makes sense to provide  this feature as well, to make it usable.