What css styles u NEED to define and style? Second question.
None, the styles in the template stylesheets are totally independant of WB and only there for the design. That's one of the great things about WB, it doesn't have it's own template system. The WB tags in templates are purely functional, and the stylesheets purely for design. Try it by removing the call to the stylesheet from a template. The site may look like a mess, but WB still functions the same.
When i add a css class from the news module wich normally is defined in the backend in my template's stylesheet it uses that style on that class? Why is the possiblity to make changes in the backend if u need to style the news module in your own template? Like post-title etc.
I'm not sure if I understand you, but many modules have their own stylesheets, but you can remove them and put all styles in a master template stylesheet if you like.