In ONevents, it's possible to define custom CSS classes. Selectors are used to access them directly. Here's a reference to the available selectors:
https://wiki.selfhtml.org/wiki/CSS/Selektoren
and also here:
https://wiki.selfhtml.org/wiki/CSS/Selektoren/Teil%C3%BCbereinstimmung
You can define your own classes in various places in ONevents, such as in the event settings under "Developer" or directly when creating an event in the space.
DE