It's just something the forum software does. Why is that feature there? Why did they choose the colors they did? Don't know.
We get questions like this a lot from a decent number of users, and it's important for everyone to understand that for the most part the forum works a lot like the Web browser you use to access it. We can install themes and extensions, and change the settings that are available to us, but the programming was done by a team of people that had a goal in mind of creating a general software package that can be used by thousands of forums across the Internet. It's not a custom job just for AARoads.
So we don't have fine-grained control over it. If there's a change that someone thinks would be a good idea, that's something we can look into, but it needs to be a setting we can change or an extension we can install. Mucking around with the actual code that runs the forum is a bad idea because unless you understand how all the various parts work, a seemingly minor change or addition can break things or cause security issues. This is why, while we appreciate suggestions or questions about forum features, we very rarely can actually implement them.
