Frontend a mess. CSS shattered

While the backend is perfectly build und works wonderfully , the frontend is a mess. I cannot find out, what ruins my css: http://joergpetersson.de/pinpoint-buchungssystem/. its a more or less naked WP-installation with Spacious Pro Theme und I only use it to test the bookingsystem.
Have you got any idea?
Thanks for your help
Jörg

https://ww.wp.xz.cn/plugins/booking-system/