First of all, login to your Onsite Pro account via your PC or Mac and click on the settings section in the main menu on the left.   Then click Customisation.



You will be able to scroll down in this section and pick the colours you wish you use on your web interface and documents.   Also its worth noting here that you can add other company information such as contact emails, your web site, and also BACS information for your customers.  Your company address and VT number will be automatically pulled through from the 'General' and the 'Accounts' sections.   


Don't forget to click save my settings button at the bottom of the page before you leave this screen.