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.
Best Answer
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.