For technical reasons, it is not possible to add background music for your whole website.
However, you can add music to single pages of your website. For that purpose, your music has to be added into a music player.
A very simple method is uploading your music to YouTube first and then add it to a page with the "YouTube" button.
1.) Click on the "YouTube" button in the editor.
2.) Then a new window will open and you can enter the URL of your YouTube music. To apply changes, click "OK".