Hi @Yan69,
footer and header are a bit different as they usually use the colour defined inside the styling in the power page designer. Can you change a colour there and see if it reflected in the page?

Other than that is what you need to do in chrome for example.
Go to your page in preview --> Right click --> Developer:

You will than see a sidebar like that. Have a look at the colourcode for the body.
In my case background-color: #FFFFFF. If you changed yours there should be your new colorcode.
If you cannot see it the change is not applied and there may be an issue with your css.
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Hope that helps.
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