In LEPTON 2.0.0 we started a new system to handle customized frontend.css of modules.
As the modified addon frontend.css were overwritten during upgrade process of modules we now use the /frontend directory in the frontend template
Copy now the frontend.css of the specified module (here news) in a directory called 'news' within the 'frontend' directory of your template and customize your file. Now the frontend.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 template that is delivered with LEPTON package.