1. You need to check with your Joomla template if the template allows you to create a custom.css file in their css folder. If the template does not support this, you can forget about this option.
2. If option #1 is not viable, create this file exactly as shown below and place your codes in that file
/templates/JOOMLA_TEMPLATE/html/com_easysocial/css/custom.css
By the way, I am just wondering why was the documentation not sought for first posting this question here eh? Because #2 has been explained very thoroughly in
https://stackideas.com/docs/easysocial/administrators/templating/adding-custom-css-codes
Is there any navigation issues with the documentation or you can't find the information that you want?