Highlight subsection of book which user currently reading at
Properly highlight which section+subsection book which user currently reading at. For example if I'm at URL https://www.git-scm.com/book/en/v2/Customizing-Git-Git-Hooks so, highlight Customizing Git -> Git Hooks when user clicks on chapters drop-down.

thanks for reporting the issue @finn0 !
would you be interested in helping fixing it? :)
@pedrorijo91, yes. But, first I need to see code-base to understand how website works and which modules do I have to look into it. And I need your help in it. Is there any documentation available for getting started quickly?
so, the website is a simples Ruby On Rails app. You can see on the README file how to setup the site locally. Under https://github.com/git/git-scm.com/tree/master/app you'll see the site source code.
I think this is the general picture. Feel free to explore a bit and come back with any questions that arise