from an amzaon forum
"This has a list of supported CSS properties
I know the list is not complete because it does not list margin-bottom and margin-left, which I have used. I don't think kindle supports margin-right and margin-top. "
centering stuff works fine, thanks, except that any book that I have converted once already has its own saved preferences, so adding extra css to default preferences does not get picked up - I have to keep adding it manually.
unless there's a way to wipe all those saved individual conversion preferences, most of which were set when i did'nt know what I should be doing!
update - google knew how:
"In any event, you can erase the saved conversion settings for books by selecting the books in question, opening 'edit metadata in bulk' (the E key if more than one book is selected), choosing 'remove stored conversion settings for selected books', and pressing OK."