There seems to be an automatic 25px margin on the <p> within the content element that we can't customize. Could you please remove this and allow this to be customized in the ThemeBuilder?
The content of a dialog can vary significantly depending on usage scenario. This is the reason why it is not exposed for customisation in ThemeBuilder.
The <p> element is used as sample content to showcase the dialog. Its 25px margin is set inline, so if you use a paragraph as dialog content in your application - the margin would not be applied. Each use case content should be customised specifically in your application.
However, I am converting this to feature request so we can track interest, gather feedback, and improve dialog content customisations.