so, first of all, i highly recommend that if you're poking around your blog in CSS and don't know what you're doing, you check out http://getfirebug.com in firefox.
then you can right click on the part of your blog you want to change and click "inspect element". it highlights the element, and shows you the CSS affecting it.
for instance, cutline's structure looks like this:
- - #content
- - #sidebar
so you can adjust the code you see to match that. #container should be width: auto, #content_box should be width:100% (since it's just another wrapper inside #container), #content 70%, and sidebar 20%.
you can apply
to whichever element you'd like to have blackspace.