Sorry to contradict Suman
DO NOT edit base theme!!!
Add the line to your own css and it will override the base css for your theme - editing base will affect every theme on your site.
If you have any theme based on a parent (and all the standard Moodle Themes as far as i am aware use base as a parent) you should not edit the css in the parent theme, but override it in your own.
It is also good practice not to edit any of the core moodle themes themselves, but to clone them or use a child theme as changes to core themes will be overwritten any time you upgrade your moodle site and are then harder to track back any changes.
Richard
Edit - came straight from gmail to post and just noticed Mary has posted the same thing