/usr/share/redmine/plugins/ is in redmine-plugin-custom-css 0.1.6+dfsg-1.
This file is owned by root:root, with mode 0o755.
..
/usr/share/redmine/plugins/redmine_custom_css/
/usr/share/redmine/plugins/redmine_custom_css/app/
/usr/share/redmine/plugins/redmine_custom_css/app/views/
/usr/share/redmine/plugins/redmine_custom_css/app/views/settings/
/usr/share/redmine/plugins/redmine_custom_css/app/views/settings/_custom_css.html.erb
/usr/share/redmine/plugins/redmine_custom_css/assets/
/usr/share/redmine/plugins/redmine_custom_css/assets/javascripts/
/usr/share/redmine/plugins/redmine_custom_css/assets/javascripts/codemirror.js
/usr/share/redmine/plugins/redmine_custom_css/assets/javascripts/css.js
/usr/share/redmine/plugins/redmine_custom_css/assets/stylesheets/
/usr/share/redmine/plugins/redmine_custom_css/assets/stylesheets/codemirror.css
/usr/share/redmine/plugins/redmine_custom_css/config/
/usr/share/redmine/plugins/redmine_custom_css/config/locales/
/usr/share/redmine/plugins/redmine_custom_css/config/locales/en.yml
/usr/share/redmine/plugins/redmine_custom_css/config/routes.rb
/usr/share/redmine/plugins/redmine_custom_css/init.rb
/usr/share/redmine/plugins/redmine_custom_css/lib/
/usr/share/redmine/plugins/redmine_custom_css/lib/hooks.rb
/usr/share/redmine/plugins/redmine_custom_css/test/
/usr/share/redmine/plugins/redmine_custom_css/test/test_helper.rb