For example some people suggest diagonal lines encourage user eye movement which hopefully leads back to the main page content.

I was wondering if there is any theory about what makes a good background AROUND your web page.

I'm not sure about that specific theory,

But I'd suggest a bright color that is not harsh on the eyes with think diagonal stripes.

I would strongly recommend leaving the bright colours for the main content itself, and using duller colours around the edges of your page - bright colours are distracting and will likely just draw the visitors attention away from the main content on the page if not used correctly.

making tiled backgrounds, hmm I think its mostly of the continuity of the design... even bright colors can blend perfectly for every site if you have continuous design flow for your background(what I mean is that you don't see any flaw or gap in between your backgrounds.

It all depends on the content. I find that very clean sites with rounded corners are the ones I stay on the longest. Bright content catches my eye. Photo's, logo's etc.

A background should be a contrast complementing element. Whether it's light, dark, bright, or bland.... as long as it seamlessly blends with the site & aids in emphasizing the content areas, you're good to go. If your Background draws too much attention away from your content, then it's time to rethink your theme.

I would agree that a dimmer, blander shade would be ideal for a background allowing the content to "POP" as they say & stick out more.

To each there own...