Editing a theme's css (width, etc)

Hi,

I’ve downloaded a theme I like, but I’d prefer if the main body of the page was wider. Is there any reason not to edit the CSS file? I’m perfectly comfortable with doing this, but just wonding does it go against ‘best practice’ when working with wp? Or is there a way of over-riding the settings in the css files that come with the theme? I have it in my head that there is.

I played around with wp early last year and haven’t been near it since, so I’m very rusty now and can’t remember a lot of what I did.

Thanks!
J.