BuddyPress.org

I don’t understand the question. To add new html, you need to add new same-named templates to your child. So to change the header HTML, add a copy of header.php from the default parent theme to your child theme and then modify. Alternately, if you only want to add HTML (not modify) you might be able to do that with a function in functions.php that echos content to an action hook in one of the default templates. Not sure how you intent to change HTML via CSS alone… other than with the content property… but I can’t imagine that working in IE.