[Kirk Y]

When you're installing modifications, you're most likely making Template Edits to your Styles. When you install new Styles, they won't have the Template Edits required by each modification to work -- as they weren't there when you made the original Template Edits.

There's no way to prevent it, just remember to do the edits needed by your mods to each new skin you install.

What Redline was saying was if you, for example, had two version of the default vBulletin Style: one was blue and one was green. Instead of uploading identical sets of images (as they're the same style -- just different CSS Colors) you can just mark one of them as a Child Style. That way, they'll share the images but have different CSS. I hope that helps you some.