avik111:
I have searched arduino.cc for references to understand interrupt, also internet for sample code, but none of them doesn't work...
One of the problems in that the NeoPixel library temporarily disables all interrupts when issuing data, then re-enables them when done. Since a lot of those functions are essentially always updating the LEDs, well then you don't have room much for anything using interrupts.
There are examples out there that use polling instead of interrupts to determine rotation of the encoder, you may want to look for those. Depending on your Arduino, you may have access to some other timers that can accomplish what you want to do as well.
Finally, refactoring some of those pixel animations to use non-blocking functions may help as well. I'd put some effort at that and see what can be accomplished there it will be particularly useful if you wat to try polling to manage the encoder.
I suggest trying Google...