You can see a calendar heatmap in the stats screen, and I believe Amboss have worked to make their functionality available on iOS by customizing card templates. Not sure if that’s still in beta; if you reach out to them they should be able to tell you more. It’s not possible to customize the background of the deck list, but you can adjust the background during review by modifying your card templates: How to Change Anki Cards Background · Shorouk's Blog