CSS Styling of EventOn not working

Hi there,
maybe someone has an idea.

I have an event and ticket plugin installed from EventOn, and as far as I understood some themes might not adapt the styles offered for the calendar. In fact the authors say: “If your dynamic styles (appearance changes in eventon settings) do not reflect on front-end, it could be that your website is blocking eventon from using wp_filesystems() to write these dynamic styles to “eventon_dynamic_styles.css”.
In this case please copy the below CSS styles and paste it on your theme styles (style.css)”

So i went ahead and placed the code into the custom.css instead of style.css (as advised by the theme’s information) – but even there the css does have no effect.

I have no cache plug-ins installed… any idea where else i should look or try?
thanks in advance
p.-