The pfUI-darkmod is a pfUI modification and an example on how to modify pfUI without touching the addon-core itself. It automatically adjusts all frame positions and draws a gradient backdrop at the bottom of the screen. This can be used as a template/instructions to your own modifications.
You need to have pfUI installed. Learn more how to download & install pfUI HERE.
*) If you have another screen resolution, you might have to adjust the UIScale on your own to have everything fit together