To overwrite any file in child theme just upload it (the file) in child theme in the same directory as the parent. In this case you have to create a “css” folder in child theme and upload the bootstrap-responsive.css file in it. Make the changes you want to this file.
To avoid the display of html tags in your topics, please read this article.
The custom css box, has no issue. Maybe you did not add it correctly. If you want to keep the current solution, ok. Or you can send us the code you tried to ad into your custom css box. So we can check if there was any mistake.
Let us know.
