Follow the tutorial:

https://youtu.be/-xIwJJQK0WY

how to use the component:

Step 1. Place the Script InstantDarkTheme_JS as a required script on your layout block! 

This Javascript file will control if the application is in a dark theme or not.

If you don't add this script to your block you will receive an error message like this: "manageDarkTheme is not defined"

You can choose the layout block or a specific block. 

Before publishing  your module you should check if your block has the JS file as a required script like the image below:

Step 2: ToggleDarkTheme Action:

In the Demo module, you can check one of the possible ways to toggle the dark theme behavior:

We created one If with a client variable to enable and disable the dark theme.