Custom CSS is found in the "Miscellaneous" category in the "Settings" section of the Admin Portal. Under the "Custom CSS" tab, advanced users with pre-existing knowledge of CSS can modify the appearance of their website. This setting allows users to write their own CSS code and insert it into the website.
We don't provide support for this feature. Therefore, if you don't already know what CSS is, or how to use it, we recommend avoiding this setting altogether and customizing your website using our many built-in tools.