When I looked at your site, you'd set it to the 'classic theme'. So that was my reference point when I was writing about #menu and #content. Which, when I edited them, happened to be in a stylesheet named 'wp-layout.css', and which is very, very close to the 'style.css' as used by the 'classic' theme.
FWIW, I think you would be better off modifying the 'classic' theme's stylesheet (unless you find one you like elsewhere) rather than the 'default', as the 'classic' is a simpler scheme altogether. Easier to connect your edits with what's happening on the screen, in other words.
For instance, if you make the edits I mentioned earlier within the 'classic' style.css, the sidebar flips over to the right, and the header extends across both sidebar and content area. Sidebar and content are not perfectly aligned at the top, but that's something you could futz around with on your own.