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. Typically custom HTML themes require an additional base theme to be added to the CanIt installation. We do not support this on our Hosted system, so it is an option that is only available to our on-site users.
See additional information specific to whitelabeling the CanIt product: