The VLC Player app for Android allows you to adjust brightness and volume by sliding your finger up/down on the left or right side of your screen, respectively.
Now through an app available on the android play store, you can bring these features to the YouTube app as well.
All you have to do is follow these steps:
With these steps, you will be able to use the same sliding features in the YouTube app, as you are used to doing in the VLC app. A semi-transparent overlay will indicate the activity of the app when the you slide your fingers, but it can be disabled in the paid version.
The paid version also unlocks certain other settings. But the choice of purchase is entirely up to your needs.