Hello Michel,
You may use Background Image or Background Patterns of your choice as follows,
Go to `Appearance > Background > Choose File (Your Pattern or Background Image)`
Upload File
You may use appropriate Display Options i.e. Position, Repeat, Attachment etc
** Make sure you are using Boxed layout of Contango as follows,
Go to `Appearance > Contango Options > Layout Settings > Box`
You may find some great patterns at,
Thanks