The hack-ish solution would be to use a PHP script to generate the style, kinda like how you use PHP to generate JavaScript and HTML code.
This’ll work particularly well for you since you have many websites with the same layout but different looks, you can get them to pass their ID or name to the PHP script, it retrieves the variable values from the data base and put them in CSS. That kinda solves the variable problem, you can have fields such as margin_left_answer, count_border_color, etc…
Or, alternatively, you can have a script that generates the CSS for you once and you upload that CSS file.