Modify Graylog2 Web interface

I am using Graylog2.2.1 in Ubuntu 14.04. I want to modify the Graylog web interface a bit, such as adding some link, some panel, images..., I tried to find the directory contain graylog web files. In /etc/graylog-server/ I can found only the server.conf, not any web files. I tried to use search in Ubuntu with keyword "graylog" but there is no folder which I can see the web files inside it. So, where is the Graylog's web UI files location?