Is there any way to customize Node-red UI(Web Portal)? I have tried to get node-red git repo and unable to find it out.
There are some limited customisations you can make to the appearance of the Node-RED editor, detailed here: https://github.com/node-red/node-red/wiki/Design:-Editor-Themes
See also this answer that explains a bit more on how to use it: How to change node-red page title