bbPress.org

this is not CSS its mostly PHP

from what it looks like just copy the templates the userpro plugin author gave you and place it a folder called bbpress in your child theme. the url to it should be wp-content/themes/avada-child/bbpress.

more info about bbpress theme compatibility

https://codex.bbpress.org/theme-compatibility/

info on creating a child theme

https://codex.wordpress.org/Child_Themes