LJK8:I was wondering if it was possible to 'abstract' from the same common properties into a single external css
Yes you can.
A web page can reference several stylesheet. Common and "static" styles can be defined in a separate file.
Then you will need to "manually" include the a <link href="...." rel="stylesheet" type="text/css"..... /> in your web pages, because it would get include automatically like it does for the stylesheet of the theme.