Hello, I searched a bit and found the solution !
You need to set the color-scheme to dark (otherwise think like scrollbar will stay white) and also add the class dark to the body
// index.html
<head>
// ...
<meta name="color-scheme" content="dark" />
// ...
</head>
<body class="dark">
Then you just need to follow what is (not perfectly) explained in the documentation (in particular the notice) :
To make it easy to integrate a light/dark theme later, what you can do is comment out the @media (prefers-color-scheme: dark) { (don’t foget to comment the close bracket) and add dark class on body, .ios body and .md body that are inside this media.
// theme/variable.scss
:root {
// ...
}
/*@media (prefers-color-scheme: dark) {*/
body.dark {
// ...
}
.ios body.dark {
// ...
}
.ios ion-modal { // nothing to change here but keep it (not tested)
// ...
}
// ...
.md body.dark {
// ...
}
/*}*/
If you removed the :root part then you can just comment/remove the media and you don’t need to add the dark class in both index.html and variables.scss. But I personnally like the fact that currently it act like I’m forcing the dark mode (and not remplacing the light one by the dark one) and if I remove the class dark in the index.html then it’ll be light mode again (+ <meta color-scheme /> to modify)
And if you want to have a nice status bar in browser/pwa (on mobile) don’t forget to set the meta theme-color ! (Advanced Theming: Quickly Customize App Colors using CSS | Ionic)
// index.html
<head>
// ...
<meta name="theme-color" content="#000000" />
Hope it help 
