This has the feeling of being a stupid question, but I’ve been trying to use the Style>Space Before… menu item to move text in an HTML box down a little to get it centered vertically. It works in Page view but appears to have no affect in Preview or when previewed in a browser.
Space before and after works between paragraph returns (correct me if I am wrong)
Is this the only text in the box? If so then I think you would be better to use padding on the HTML container set as custom just top and bottom to place your text where you want it.