i know that but where is the ccs file?

Inspect element gives you a path to the css file for a given element. Just hover over the link to the right of the css. Maybe this will help, or maybe you've already tried that.

Thanks!! 

but that css is in the static folder...

Jordi, yes, you are right. After last update all admin panel files were moved to /ow_static/plugins/admin/ folder. So, to change admin panel CSS you will need to modify /ow_static/plugins/admin/css/admin.css file.
After changing the code make sure you've cleaned browser and site cache. To clear site's cache enable DEV_MODE. Follow the instructions: https://docs.oxwall.org/plugin-tuts:enable-debug.

Thanks taissa !!