You can change each and every dom element css in developer portal using CSS & Custom Themes. You need access to the code base of developer portal. You also need little bit of Drupal Themeing expertise to work with themes and change css.
Please raise a support ticket
here to get access to pantheon hosting platform where our cloud developer portals are hosted.
Once you get access, create your own sub theme out of Apigee Responsive theme and start adding your custom css in sites/all/themes/YOURTHEME/css/style.css file.
Check doc here on building a new sub theme in Apigee Developer Portal