Where is Safari Reader CSS or HTML in El Capitan? In OS X El Capitan, Safari Reader View file for modifying the CSS is no longer found in Safari application and in /System/Library/PrivateFrameworks/Safari.framework/Versions/A/Resources/.

Where is it located?

Is it locked by System Integration too?

If there is no way to modify, I think in downgrading to OS X Yosemite.
I tried mdfind -name Reader.html and on my copy of El Capitan it yields the following path:

/System/Library/PrivateFrameworks/SafariShared.framework/Versions/A/Resources/Reader.html 

So it looks like the file has indeed been moved.