How to Change Your Inbox Theme in Gmail
If the plain white background of your inbox is getting boring, you can change themes to give it a more interesting look.
A themes gallery appears, showing a variety of themes to choose from. The top of the gallery shows some curated images you can use as a background.
The background of the inbox changes to the picture you chose.
You can also create a theme from a larger set of photos, or from photos in your Google Photos library, instead of choosing from the gallery.
Here you can choose between sources for additional photos
The photo is set as your inbox background.
Once a photo has been selected, you can customize some additional options.
The theme is updated and applied to your inbox.