As far as i know there is no styling for Rubrics in any theme, as this is styled automatically at sourse. You will need to look in Moodle/grade/grading/rubric/form/styles.css for all the styles used.
To save you looking for this I have attached the style sheet here for you.
There have been some changes made recently in Rubric, so it might also be worth checking MDL-30780 for this.
Mary