Good evening James and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
We are sorry to hear about the challenges you are experiencing with CSS changes.
I would love to help you with this topic.
As a first troubleshooting step, could you please provide us with your complete system information in a private reply using the instructions found in the following link?
https://theeventscalendar.com/knowledgebase/sharing-sys-info/
Secondly, there is a fairly strong chance that the issue can be fixed by going to Events -> Settings -> Display and changing the settings for the “Default stylesheet used for events templates” option.
Let me know if that helps.
Have a great day!
Geoff B.