Looks nothing like the demo website on my installation

I'm aware for most of you this will sound like a stupid question, but i'm rather new at Joomla

I installed JEM on my website but it doesn't look anything like the demo site. This shall probably be caused by my template css-settings (Helix Ultimate bij Joomshaper).

In the settings i find some custom css stylesheets which are all set to "No". I've tried setting them to "yes" one by one but this doesn't change much.

For example the "add events" form looks totally messed up (see attachment).

Is there some way that i can fast achieve that on y website it looks like the demo website, or do i have to manually change alle css-settings (which would be very time consuming)

Hi, welcome in this forum where users try to help other users.
I hope (because you gave no information) you are on Joomla! 3.9.1 and JEM 2.2.3.
Normally you should look for help, which you can find in Backend JEM control panel >help.
Because you changed a lot without knowing, it can be that you have to uninstall JEM, but to uninstall the JEM database too, you should first go to Backend >JEM>Control Panel> Settings>Global Parameters>Cleanup DB on Uninstall=Yes
Then uninstall all JEM. And Reinstall.
To understand the JEM system I propose you to use the protostar template. So go to template and activate protostar.
In backend JEM control panel you can choose load sampledata. Then look and consult the help to learn more about JEM.
After you can adapt for your template.