How / where to add custom css?
Please add your custom CSS code into the next field from WordPress customizer:
WordPress Admin > Appearance > Customize > Additional CSS
By using this method you can be sure to not lose any custom code on theme update. Alternatively, you can use a dedicated plugin for this job. For instance: Simple Custom CSS and JS.
Our theme supports a child theme and we have an empty child theme included in the full theme package ( see Premium Design Assets > seowp-child.zip ). Child themes are better for complex customizations or when you need to change PHP code as well.