Many moons ago I changed the original color scheme to what you see in the picture..... hideous at best.....
I am not sure where in the color palette to revert the gray back to the default blue.
For future reference, create a default XenForo style and hide it from guests. That way you can use it for things like this in the future and to troubleshoot.
For future reference, create a default XenForo style and hide it from guests. That way you can use it for things like this in the future and to troubleshoot.