Do you need HTML or CSS skills to design templates in iText DITO?

No, the iText DITO template editor is designed to be used by anyone with basic Office skills.

However, more advanced users can also add either embedded or external stylesheets to the editor, meaning CSS styling can be easily applied to any templates you design. 

For example, you might want to use consistent formatting or company branding for your documents; this can be easily achieved by applying a CSS stylesheet to your templates.