Hi @amirdire,
Yes, you can override the backend templates as well as inject CSS/LESS code into the backend. A theme plugin has a templates directory. Though our documentation focuses on overriding the frontend templates, the directory can also override any backend templates.
https://docs.pkp.sfu.ca/pkp-theming-guide/en/theme-backend
Also, the techniques for modifying a parent theme’s stylesheet can also be used to modify the backend stylesheet:
https://docs.pkp.sfu.ca/pkp-theming-guide/en/child-themes
When modifying that stylesheet, the stylesheet name is pkpLib and the context is backend.
Beware that modifying the templates or stylesheet of the backend can break the editorial backend. We generally don’t recommend it unless you have resources to develop and test carefully.