Actually I’m with xoxo on this one: I’m just plain lazy, know only JEXHTML (JE=Just Enough) and don’t know enough CSS to pull that one from the top of my head. (Thanks for the CSS Viper007Bond.)
As for forcing widths, while it may just be a matter of preference, it may well also be a matter of necessity. If your writing style is short-and-sweet then a narrower theme will make it seem like you have more content. Nothing I hate more than to write a tiny, sidenote-type post then have it stretched over 1600 px. *ugh* Makes me wanna puke when I see that, and most writters react to it by wanting to fluff their articles with useless material, thereby degrading the reading experience, just so it doesn’t look like total crap.
It may also be necessary in cases where you want to have mutiple pages per post, but only want up to a certain length. Guessing that someone’s going to look at your site at “1024×768” is hardly the same as forcing them into it. At least when you force them into it, it’s not as bad, even if you do tick off the 800×600 and “small floating browser window” crowd. (Besides 1024 = average anyway)
Here’s a good example: You lead off all your articles with a picture wrapped by text, except this time you only write a short paragraph. You place the picture there as normal and think, “this looks just fine in my screen 1024×768 screen through my Firefox on Linux browser.” Then you go to your friend’s house which runs his resolution at 1400 x 1050, uses Opera and runs Window. When you look at the site you say “Oh no! now that picture pushes all the text in the next article down! This looks like crud now!”
I’ve worked as a writer for a number of websites and we’ve gone through entire redesigns in good part for that very reason. On the downside, they tend to alienate people. (Can you imagine my friend, running his resolution at 1400×1050, being subjected to a forced resolution of 800-width? Talk about wasted real estate!) Generally, however, controlled-width themes are much easier to control, aesthetically speaking.
*chink chink chink* My $0.03 (adjusted for inflation)