Hi Suzanne,
It is not possible using CSS only. CSS are changing designs. There are special case that we can change a text using CSS but not at this point. The change you want is possible using customization. Yes php code.
Here’s a guide from bbpress:
https://codex.bbpress.org/themes/theme-compatibility/getting-started-in-modifying-the-main-bbpress-template/
The specific template that contains those titles can be found on this folder:
wp-content\themes\pro\bbpress
These files
loop-forums.php
loop-topics.php
You may customize those template on your child theme. Create the same bbpress folder inside your child theme, copy those templates and then customize the content as needed.
Note that we can only give you a guide point you toward the steps you may take but the actual customization is outside the scope of our support. If in case you are not familiar with php, it would be best to consult a web developer.
Hope this helps.