Is there anyway we could use CSS to assign design rules to all objects displayed in dashboards (tables, charts, filters etc)? Templates and Themes are good but the rest of the sites we are working with are CSS based and this would simplify things.
Managing all Dashboard object with CSS
You can customize almost everything in Dundas BI. For your particular needs, you are looking to create a CSS override file. You should be able to find all the information you need in the White Labeling article.
I know you rather use CSS but it may be easier to create a theme and set that as the default theme at the entire instance level (meaing every new object created will use that theme).