/usr/share/redmine/plugins/redmine_custom_css/assets/stylesheets/codemirror.css is in redmine-plugin-custom-css 0.1.6+dfsg-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/redmine/plugins/redmine_custom_css/assets/stylesheets/../../../../../javascript/codemirror/lib/codemirror.css