Two ways to theme: the easy way is via Setup : Theme Customization. This lets you drop in a logo file to change the one in the corner, customize the colors and even override some CSS options at the bottom.
The more advanced way is to come up with you own HTML/CSS changes using our Theming and Branding Guide as a guide. This requires an experienced HTML/CSS developer. On the hosted service, it is necessary to work with our devel. team to integrate your changes but it should be possible.
There's no way to remove the CanIt references from the interface unless you want to go the advanced HTML/CSS changes route, in which case it will be necessary to have your experienced CSS/HTML developer discuss that option with our devel. team.
On Hosted CanIT, the Theme Customization settings can be applied to the login page of the WebUI by using Setup->Templates and setting the base URL to: https://antispam.roaringpenguin.com/c_example-org, where 'example-org' is the name of the realm where the customizations have been defined
Note that the 'c_' prefix and an enabled entry in Setup : Theme Customization in manditory for this behaviour and that it may take up to an hour for the link to become active.