Removing RSVP buttons via css: Which stylesheet to edit?

I added the css .wpmudevevents-buttons { display: none;} to my child theme stylesheet but it had no effect.
The RSVP buttons on events remained visible.

I eventually edited the CSS front.css within the css directory in the plugin and that made the buttons invisible.
I had also added the code to the default template> fullwidth>events.css. That didn't seem to affect the display.

My questions:
1. Would the more usual place for this css edit to work be in the theme css, in this case the child theme css?
2. Is it possible to have a custom css sheet within the plugin that I wouldn't have to remember to edit the stylesheet directly after each update?

Really wish the display of RSVP buttons was an optional setting but willing to work with this as long as I know what to do.