Body, Text & Links sets the default style for the page. Anything that doesn't have an override gets it's style from this default. Each area of the theme has a place to add style for that area.
If you want to style the center column (the posts) you could do it at ATO->Style & edit CENTER COLUMN->Center column style. But it gets better, you have even more control. The center column is made up of 'post containers', ie inside the center column are post containers which can be styled (two types 'sticky's and normal posts) and each 'post container' is made up of the container, kicker, headline, byline, body and footer (see ATO->style POSTS & PAGES) each of those areas can be styled independent of the others.
HEY, shake the glaze out of those eyes (just kidding
) This arrangement gives you an unprecedented level of control, or you can do as little as you want.
If the style in Body, Text & Links is good for the sidebar, you can just add styling at ATO->Style & edit CENTER COLUMN->Center column style fior the center column
(BTW, you can style each widget in the sidebar differently...but that's a story for another day)