What am I doing wrong here ?

[fathom tundra]

So i have been trying to get this to work. it is to change the background depending on what theme is in use (dark/light).

:root #app-mount .theme-dark { background-image: url(https://i.imgur.com/La3SG7L.jpeg) !important; }
:root #app-mount .theme-light { background-image: url(https://i.imgur.com/pNaBdEQ.jpeg) !important; }