Insert the code in the "Custom CSS code" textarea, located in the Dashboard > wpDiscuz > Settings > Styles & Colors admin page.
Don't forget to delete the caches before checking.
Also, please try in incognito mode if you're checking on mobile devices. Because mobile browsers have a hard cache, you'll not be able to see the change immediately.
In case you want to say thank you! 🙂 We'd really appreciate if you leave a good review on the plugin page. This is the best way to say thank you to this project and the support team.