I'm looking for alternatives to WebIOPi. I need to be able to control PI's GPIO through a UI displayed in the browser (with a Customizable UI ofcourse)
WebIOPi provides a REST interface for GPIO control. You can write any UI you want on top of that.
Try BerryIO. It's an outstanding piece of software.
Try WebGPIO at Github and still active in this 2018 :
The WebGPIO is Flask based. So you need to have Flask installed. Also install the pyyaml for the yaml framework to run WebGPIO.
pip install flask pyyaml
Beside Flask, you can als try to use Bottle. Both Flask and Bottle can be used to run Python script via web browser (HTML).