Well try using your browsers Web inspector that will guide you through your code and gives you a visual image of where and what you can change because every site can be different.
This is an old video but I think every browser today has an Inspector inside it by default. It gives you an idea of how you can spot some HTML classes and what you should target to change.
Btw, if you have copied the default bbpress.css stylsheet from the plugins folder to your own theme ( wp-content/themes/your-theme-name/css/ ) folder you can edit and customize that new bbpress.css file. You don’t have to place all your custom CSS inside the style.css – just customize the bbpress.css inside your theme folder.
Right now you’re working too much with the !important; attribute just trying to overwrite default stylings while it’s not needed if you simply customize the bbpress.css stylesheet.
I’ll try this at home for sure(Right now i am working).
I’m using a child theme, and don’t know too much about bring another css style to it. maybe it’s simple, maybe not. i will see this later for sure (about !important and how to import css of other plugin)..
I really appreciate your help and come back with a answer soon.