I could not find one working in https://userstyles.org/.
I use Firefox and I'm open to use any extension allowing the restyling of a website, like Stylish.
I know "less ugly" is subjective, but I'm interested to try whatever attempt someone would have shared for that.
You could use an image uploaded on Google Photos as a background:
Use one of your photos as your background
1. On your computer, open Gmail.
2. In the top right, click Settings Settings.
3. Click Themes.
4. In the bottom right, click My Photos.
5. Choose a picture, or click Featured. If your picture looks blurry, try uploading a larger sized photo.
6. Click Select.
7. Near the bottom, make changes to the theme by using the Text Background, Vignette, or Blur buttons.
8. Click Save.
