To customize your Online Ordering site in the Back Office, see:
Setup > Online Ordering > Design
You are able to add your business logo, a header image and set your background and theme colours for the site. This allows you to create a site which fits with your brand.
If you wish to have a more sophisticated design, you have the option to create a custom designed initial ordering page. The initial ordering page is the page shown below. More details about this are available in:
Setup > Online Ordering > Link for site and integration