The CSS codes should be added in the "Custom CSS Code" textarea, located in the Dashboard > wpDiscuz > Settings > Styles & Colors admin page.
Regarding the second issue, we'll need your website URL to debug the CSS conflict.
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.