Hello infinity-studio,
Welcome to the GDevelop forum and thank you for your topic request.
So you know, customising leaderboards with CSS is already possible with a Professional Sub. 
Also, please take the time to edit your topic to have only one request. As explained on ⭐ Welcome to the User Feedback channel! the forum rules ask contributors to keep only one demand per topic so we can track the votes and the discussions related to that specific desire or friction. That way we can tag them as “in backlog”, or “done”.
Also, feel free to take a look at the Topics tagged leaderboards. You will see that some of the things that you request are already identified by other users. Consider voting the original message and comment to give it more traction.
I’ll wait for your changes before processing and tagging this request.
If you didn’t manage to make the changes I will archive this request to keep things organised.
Thank you for your help.