Changes for the Ephox EditLive RTE can easily be done by modifying settings in the configuration files located here:
/customization/javascript/ephox/editlive/
For example changing the default font ( which has to be done in the CSS file ).
After some modification you only have to clear your browser cache, reload the current page ( e.g. the Wikis ) and open the editor again and voila => all modifications are displayed.