This is an easy-to-use extension for YouTube that lets you easily switch between the two themes. You can set this as your default theme so you don't have to remember to toggle the theme every time you launch the app. It has an interface that is easy to use and you can even choose to turn on and off the dark theme in order to keep it light-weighted. It's available for desktop browsers and for mobile browsers and the Android version is available as an app as well.

In order to create a dark theme for your YouTube pages, you can use this extension. You can set the color scheme by customizing the default color scheme of the page. If you want to change the font-size, you can do it by editing the style and adding a font-size change.