.css, .sass or .less
Let's see, as an example, the different ways you have to change the backgrounds image of the login page.
First let's download a background image an put it in our public directory:

Let's apply this image to the body using plain CSS
body.kcBodyClass {
background: url(/background.png) no-repeat center center fixed;
}
We import the StyleSheet:
import "./main.css";
import { Suspense, lazy } from "react";
// ...
Result (see testing your theme):

If you prefer, you can also move the background.png image from public/
to, for examples, src/login/assets/background.png
and reference the image with a path relative to the CSS file, in this case it would be:
body.kcBodyClass {
background: url(./assets/background.png) no-repeat center center fixed;
}
In the following video I show how to load different background for different page and how to create theme variant.
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