Customize each list by choosing a different background image. Just select the three dots in the top right and then Choose Theme. Then just pick one of 5 illustrated themes or a solid background.
Customize your lists with colors. Just select the three dots in the top right and then Choose Theme. Then pick one of 5 colors. The color you choose will also be shown in the sidebar next to the list's title, so you can organize all your lists by color.
Just add an emoji to the beginning of your list's name and the emoji will be displayed as the list's icon in the sidebar.
Source: Microsoft