On Print-CSS.rocks every example has an HTML file and includes as global css file from inside the HTML.

In some cases it is necessary to provide a converter specific CSS file (e.g. a particular converter needs a specific CSS for a particular feature like float: prince-column-footnote).

Although I can specify a custom CSS file as external stylesheet using the cmdline option -s
however this file is loaded before default stylesheet specified inside the HTML file:

prince index.html -s prince.css -v

prince: warning: disabled parallel downloads: failed to initialise locking for OpenSSL
prince: loading document: /usr/local/lib/prince/license/license.dat
prince: Loading document...
prince: loading HTML5 input: index.html
prince: loading document: index.html
prince: Applying style sheets...
prince: loading style sheet: prince.css
prince: loading style sheet: styles.css
prince: loading style sheet: ../styles/a4-landscape.css
prince: loading style sheet: ../styles/common.css
prince: Preparing document...
prince: Converting document...
prince: used font: Lato, Bold
prince: used font: Lato, Regular
prince: loading hyphenation patterns: /usr/local/lib/prince/style/../hyph/hyph-en-us.pat
prince: Finished: success

I would be more helpful if you could load a custom CSS after the default CSS of the HTML file in order to override the default CSS behavior.