How to customize android operating system of my smartphone?
Check out this XDA Developers subforum: http://forum.xda-developers.com/lg-l90

That's the general discussion forum for your phone, and you can find lots of fun stuff there including mods, themes, guides to root and do other things, custom ROMs, kernels, and more. If you want to play with or change your operating system or its features in any way, XDA is the place to keep a lookout in. But make sure you read posts thoroughly to make sure that you know what you're doing and so that you don't end up breaking or bricking your phone. 

As for your specific request, this thread in the L90 forums will get your phone rooted quick and easy as long as you follow the instructions well. Make sure you get ADB and download all the files prior to starting the process, and also, please don't jump into it without making a backup.

Good luck!