You can't override that with css. It is a THEME option.

@123toid  ... also, I am sorry if my suggestion did not work out for you.  As evidenced by my screenshots, it shows that same CSS Code in action - it is the CSS Code that I used / use on my forums to override the space issue on my forums.  When I started, the "space" above also was an issue for me, so I "hid" the Forum Title.  I also tested and use that code that I shared here, it might help others as well. That same CSS Code has worked for me thus far. But I guess I need to also put a disclaimer - it worked for me, it might not work for others. (I created my own WP theme, so I know a little bit about WP themes and templates).  All I can do is try to help and make actionable suggestions and not just state opinions.  And thanks for appreciating my effort at trying to help.  🙂 Good luck and God Bless You! 🙂