You can change the theme in Gmail web version (PC) and here's how:
Sign into your Gmail account. Click on Gear icon > Themes.
In the "Pick your theme" popup, you can see the themes now. Select the theme you prefer by clicking on it.
If you are not happy with the themes in the "Pick your theme" popup or want to see more themes, then follow the below steps:
You can notice "More Images" option within the "Pick your theme" popup. Click on it, and you'll be taken to the "Featured" section of "Select your background image" popup, where you can see a lot of themes.
Select the one you prefer by clicking on it and then hit the "Select" button. After that, follow the below steps.
Now, you can see that the theme you've selected got applied to your account.
Also, you may see these three options at the bottom of the popup which you can use to customize the theme's look:
Text Background - Click on this option, and you'll be shown two options: Light and Dark. Pick the one you prefer.
Vignette - Using this option, you can add Vignette.
Blur - As the name indicates, this option is for blur.
Note: The above three options are optional, and it'll not appear on all the themes.
If you don't like the theme you've selected, then select another theme, customize it (if you wish and the three options are available) and see how it looks.
If you are happy with the selected theme, click on "Save" button.
That's all. All done.
If you want to set your picture as the theme, then here are the steps you need to do:
The image you want to set as the theme should be in the Google Photos. If the picture is already there, move ahead to the below steps. Else, upload it there and then follow the below steps.
I hope that this guide is useful to you. If it is, share this guide with your friends.
How to Change Theme in Gmail