If you are facing the query of how to device then you need to start by signing in to the Gmail on the web browser. After that, click on setting panel or gear icon and then choose themes. Here, different themes will appear, and then scroll below to find the native Gmail theme, dark theme, or any other preset. From here, you can upload from my photos at the bottom. You need to choose the desired image or theme and then click to save. Lastly, you can enjoy using the changed Gmail theme.