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Access to body style element using callback

I’m using Dash in combination with flask: login, responsible toggle sidebar, graphs…
Now I want to change the overall background depending on the page content. For example I see a full size image on login page and a white background on page1. To add a background I use the css-file and I’ve defined it in the ‘body’ section. Is there any possibility to access this body element using callback? e.g.:

Output(“body”, “style”),
[Input(“url”, “pathname”)],
def change_bg(pathname):
if pathname == “/” or pathname == ‘/login’:
return {‘background-image’ : ‘url(/assests/image.jpg’)}
return {‘background-image’ : 'url()}

Thanks in advance