If you don't know How to Change Gmail Theme, then you can follow the steps mentioned here. Open Gmail and select the Settings icon, which looks like a gear. Then, scroll down to the Theme section and select one of the pre-made themes or select View all. If you chose View All, scroll through the photos and choose a theme to preview it. You can make changes to the themes by clicking Text Background, Blur, or Vignette. Select More Images to see even more background images. Lastly, select Save to apply the new theme to your Gmail inbox.

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