This is what I have so far:
RewriteRule ^([A-Za-z0-9_]*)/?$ index.php?page=$1 [NC]
RewriteRule ^([A-Za-z0-9_]*)/([A-Za-z0-9_]*)/?$ index.php?page=$1/$2 [NC]
The problem that you are experiencing is not that the mod_rewrite is being applied to css files, but rather that the paths in your html to your css is relative. The browser is simply requesting the css from the correct relative path. You really have two options.