/usr/lib/python3/dist-packages/matplotlib/backends/web_backend/jquery/css/themes/base/jquery-ui.min.css is in python3-matplotlib 2.0.0+dfsg1-2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/python3/dist-packages/matplotlib/backends/web_backend/jquery/css/themes/base/../../../../../../../../../../share/javascript/jquery-ui/css/smoothness/jquery-ui.min.css