Because the colors match accordingly to the forums. Changing it would just look weird.
You're doing something wrong Nova, this ain't an Opinion Thread (It's posted inside MyBB Support). Regardless of your opinion, the topic is for asking Help on achieving something, if you like it or not is a thing to keep to youself.
The reason i want to do so is to add a non invasiveOverlay on the right side of it (Where it is blank).