How can I override backend templates (OJS 3)?

Yes, I created a handler in /ojs/lib/pkp/pages/admin/AdminOneContextHandler.inc.php and modify the function name. contexts by mycontexts.
I have a new error:
This is my URL
http://localhost:800/ojs/index.php/index/user/authorizationDenied?message=user.authorization.roleBasedAccessDenied

My log:
"GET /ojs/index.php/index/user/authorizationDenied?message=user.authorization.roleBasedAccessDenied HTTP/1.1" 200 8838

Image with error:
ojs3

I am logged with my admin user.
I have the same error as OJS 3 - Authorization Denied
I installed in localhost with xampp
mysql --version
mysql Ver 14.14 Distrib 5.7.22, for Linux (x86_64) using EditLine wrapper

I cannot access to show the page with all the journals to edit like this:
ojs4

Regards.
xavi