Sancta Simplicitas is a WordPress theme that uses YUI CSS utilities: reset, base, fonts, grids. It's very minimalistic in a sense that it's pretty much all white and simple. Hence the name. The theme is probably not usable by itself, but it's a base on top of which you can create your own themes.
When creating a new WordPress theme, people usually take the default Kubrick theme but I personally find Kubrick too much. So Sancta Simplicitas has a very minimal stylesheet and is based on the WordPress classic theme.
The fact that it uses YUI grids makes it trivial to tweak: fixed size vs full width, width of the sidebar, position of the sidebar. Basically right after the
<body> there's this:
<div id="doc3" class="yui-t5">
Changing the id
doc5 will give you different widths of the content. The default
doc3 is full width. Then changing the class name
yui-t5 will give you different position and width of the sidebar.
yui-t3 put the sidebar on the left hand side,
yui-t6 place it to the right. You can go even crazier from here, nesting grids to get a two-column sidebar and so on, it's really easy with YUI grids and the docs are here.