This is based on 1.15.x but it should work also on 1.16.x

I have added the css file here:
https://github.com/theking81/osTicket-Dark-Theme-Admin

This is a small preview:

IMPORTANT INFORMATION:

Please note this css has a wide screen page.
You can add the wide screen page based on this topic:
https://forum.osticket.com/d/93476-resolved-widen-osticket-page/4

If you want the BOX version, please replace:
on line 129: from 100% to 960px
and on line 729: from 100% to 960px

Thank you