Design, like programming, has a whole truck load of principles.
One of the most prevalent theories is the rule of thirds.
It states that if you want to make something interesting looking, all you have to do is divide a picture into nine squares (3x3 grid) and align the image's subject along two of those lines. If you were to simply center the subject of a picture, it would inherently be less interesting than if it is along one of those lines.
This is the reason grid systems in CSS frameworks are so popular. Grid systems allow you to easily divide a page up into sections using predefined classes. Most of them, like Bootstrap and Foundation, allow you to create 2 to 1 ratios to create visually appealing content sections taking 2/3 and a sidebar to take the other 1/3,
I encourage you to checkout http://expo.getbootstrap.com/ and see this principle in action. The website, http://www.leodislager.com/, is one of the sites up there right now that is a fantastic example of the rule of thirds. It was mind opening for me to realize that design techniques like this could be described and reproduced logically.
Check out Part 1 if you missed it.