Try this CSS code. Put it in Dashboard > Forums > Settings > Styles Tab > Custom CSS Code textarea, save it, delete all caches, and use private/incognito tab of mobile device browsers to avoid those hard caches and see the changes:
In case you want to say thank you !) We'd really appreciate and be thankful if you leave a good review on plugin page. This is the best way to say thank you to this project and support team.