AFAIK topic-footer-buttons is not present in the page of a long topic until you reach the last set of posts where a footer is appended, so CSS alone won’t do the job.
It’s a bit of an odd one though, responding to the topic ‘at large’ implies you have read it. If you really want to reply to the topic you can click the reply button on any post, then change the reply action to ‘Reply to Topic’:
It’s theme-component territory, because you’re making structural changes to the layout. Moving this from support because your CSS proves you’re doing everything right to control the element once it’s actually present within the page.