Take a look at $PLATFORM_TOMCAT_HOME/webapps/eXoResources/groovy/organization/webui/component/UIOrganizationPortlet.gtmpl. This file contains the template definitions of the Organization portlet.

Create a UIOrganizationPortlet.gtmpl file and put it in custom-extension.war!/groovy/organization/webui/component.

Copy the existing content from UIOrganizationPortlet.gtmpl of Step 1 into your custom-extension.war!/groovy/organization/webui/component/UIOrganizationPortlet.gtmpl, then modify your file, for example change the color of text and background on the toolbar.

Refresh the browser if you are running eXo Platform at the developer mode. You will see your modification take effect on the Organization portlet.