I am by no means a CSS-guy, I only do it when necessary (i.e. when prototyping something) but this approach feels so wrong to me: It basically moves the styles

I am by no means a CSS-guy, I only do it when necessary (i.e. when prototyping something) but this approach feels so wrong to me: It basically moves the styles from the "style" attribute to the "class" attribute. How does this approach deal with themes? How can I redesign a page without editing all HTML?