allow user to set some css settings through UI
Since the solution I reference in my first answer was created, I have learned new things. If I were to do the same things again, I would base it on a similar principle, but I would use Panels per pane "CSS Settings", to add CSS classes instead. See: How does one theme a panel (or add a style)

This should allow for much broader customization of content.
A problem with this approach is that Panels UI is hardly "customer friendly".
I would solve that by a custom form, on a dedicated admin page, that simply loads the pane settings from the database, overwrites them, and saves them back.