I'd love to be able to define and use a custom stylesheet. It would make styling a lot of similar elements much faster.
For example, if I have a list of cards that each has a Header, some text and an Icon, I can set a reference on each one to StyleSheet.Sheet2.Card.Header
StyleSheet.Sheet2.Card.Body
StyleSheet.Sheet2.Card.Icon
Right now, I'm having to go through each one, and manually refer each element of the style to a "source" element in the page. eg:
PaddingLeft = Card1Header.PaddingLeft
... and so forth for every style element of every item of every page.
We are working to expand upon options for theming. This may, or may not, include integration with CSS files, but it will facilitate your theming goals.
Thank you for your patience as we evolve the product,
Audrie