(note - this is work in progress)
You can do basic customising of CGI:IRC by using the options in the config file, but to intergrate it into a website then there are more things you might want to change. The colours are controlled by style.css in interfaces and the format you use formats/default by default. The CSS lets you control how all the parts apart from the main frame look, the main frame is controlled from formats.

The login form can be copied, see this for a basic login form. Alternatively you may use a link (view source of that page and adapt it to your needs by changing the server and the channel (twice!)).. Another option is to adapt the output of Blitzed's addchat page. Once you have created this page you can add the line form_redirect = http://location/.. to the config file so normal vists to the login form will be redirected to this page.