Custom CSS Events Calendar

Sorry to ask this stupid question, but I cannot find a working solution.
I would like to change the round-shaped border (JEvents style is "flat" I think) of the events calendar and the navigation bar therein to rectangular shape, i.e. for example in

#jevents_body {

    width: 100% !important;

    border: 1px solid #DDD;

    background-color: #F7F7F7;

    border-radius: 8px;

    padding-top: 10px;

    padding-bottom: 10px;

}

Should become border-radius: 0px.
I tried to add the change to the Custom CSS but it does not work, because obviously I miss the correct syntax somehow.
Could someone help me, please? I am using Joomla 4 and BS5 if this is of importance.

Thanks and best regards,
Bernd