How do you modify the default style ? We still have problems with that one .. less than with others though lol.
You can't modify the default. I just looked and it looks like the style itself is fine.
If you mean there are images that are red x's they arent part of the stylesheet.
You can fix them if you dont wanna wait on SMF (and it'll make this not happen again btw) by doing this:
Go to Admin > Manage Styles > Customize Images
Check the urls for the images there.
Any that start with
http://www.smfboard.com change the "www" in the urls to "images" so they start with
http://images.smfboards.com