Firstly, I congratulate you with using the override system. Secondly, that you are starting from scratch.
Basically with the default template, the colors and other things are left as such that you would have to experiment and learn your way around. The easiest way for you is to compare right to left.
You will first see that your navigational columns(side rows) have different background colors. As you are using a white background, change the color declaration of #navColumnOne to white(#ffffff).
Now, you will see an extra margin still exists between side boxes on that left column, but at least it's white. Go back into your stylesheet and compare between that one and the right one. Maybe you'd like to margin your right side as the left side. So, you've probably noticed that margin is coming from .leftBoxContainer and you can't find the .rightBoxContainer... no worries, copy and paste that code from the left container and change the title to .leftBoxContainer.
It's all trial and error, but it's a good way to learn the basics. :smile: