The Main Theme - this is a full with Theme, hence content placement is based on the size of the users screen.

Acceptable as external site

The Documentation Theme - this is a full with Theme, hence content placement is based on the size of the users screen.

General not very acceptable as external site, unless documentation is the main requirement.

This sidebar can be configured as a page, which adds flexability a lot, se Placing Resources for ideas of placing the sidebar page.

The Easy Reader Theme - this is a fixed with Theme, hence content placement is pretty controlabled, making it my favorite as an external site.

Review CSS For Confluence 4.X for setting the width.