central layout themes

Hi there
I am building my blog with the theme Platform by PageLInes but I don´t know how to set up a central layout with it (sorry I am a web newbie and I don’t know the exact term for this sort of look where a simple page stays centered in the middle of the screen over a background)

Does Platform allow to set up a central layout design? How do I do it? (pages seem to extend with no limits at the right side, and I want a clear neat rectangle for my content in the middle of the screen…)

Otherwise, Can anyone point me to central layout looking themes, versatile and highly customisable (custom header, custom menus, etc…)

Thanks a lot for your help