In LEPTON 2.0.0 we started a new system to handle customized backend.css of modules.
As the modified addon backend.css were overwritten during upgrade process of modules we now use the /backend directory in the BE-Theme.
Copy now the backend.css of the specified module (here news) in a directory called 'news' within the 'backend' directory of your theme and customize your file. Now the backend.css of this addon will not be overwritten by upgrade process of the news module.
This will work with any other module the same way.
Examples can be found in the LEPTON BE-Theme "algos".