You cannot use custom CSS in the theme rule, you should use the supported design tokens to customize the Constellation UI theme.
Is the colour that you are trying to set for the header different to the light / dark defaults for the OOTB light / Dark themes? What colour are you wanting to set?
@Marc Alderman Is there a way to define new CSS fields other than existing design tokens. For ex i want to change my cursor icon for all buttons to a custom one with URL option. I could not see an option of adding custom CSS that holds all common styles or modifying Rule-UI-Themes throw error on invalid JSON .