CSS is available from version 1.1, and custom HMTL from version [to be added].
While we provide the possibility to add customization as html and css, customizations made by using html or css are not supported by Refined.
Customize using html or css
Go to Administration > Add-ons > Theme Configuration and click on tab themes. At the bottom of the page you can expand to see the CSS interface.
Please note that this is not supported by RefinedSupport, so tread forward with caution.