Fix for firefox scrollbar thickness

[NikPiermafrost]

Hi all, it's really the first time i ever done some contributing. I pulled the webapp and published a docker image. Since i don't really like firefox thick scrollbars, i tought to fix them and show you the results.

https://tg-nicolapi.ddns.net/

Is there anyone who can help me with some basics step to let me start with contributing?

Thanks for any help!