I have a website that has an index.php, a few simple .html pages and one .php page (contact form) as well as a .htaccess file. On my website I have buttons that point to the
If the stylesheet is being included using a relative path, then that would be the problem. Try using: /path/to/style.css rather than relative/path/to/style.css
So if your stylesheet lives in templates/style.css and you reference it as such, when you rewrite the URL http://www.site.com/index.php?page=welcome to http://www.site.com/welcome/, the stylesheet's path is relative to that:
http://www.site.com/welcome/templates/style.css