The first step is to sign in to your AssetPot account as an admin user. You can sign in here
Once you have logged in, click on 'Manage Repository' on the menu at the top.
On the 'Repository Preferences' page, look down for the section called 'Theme Colours'. In the box here put the hexidecimal code of the colour you would like to use. You can find the hex colour you want at www.colorpicker.com