How can I change the background color in the game html5, if I do not want to stretch the image to the whole screen, for example I want to change the black color in the picture 
to blue, how do I do it?
How can I change the background color in the game html5
Try Default Clear Color in Project Settings or Background Color in the Export Presets. Gotta be one of those two I'd think.
Go into Project Settings -> General -> Rendering -> Environment -> Default Clear Color
If you want to change the html background colour, you can change the generated html by modifying the body background-color (see the default html file) (the default style is black)
Manual page here: https://docs.godotengine.org/en/stable/tutorials/platform/web/customizing_html5_shell.html