.wpmeetupactivity-setup-fieldset {
    border:1px solid #aaa; 
    border-radius: 5px; 
    padding: 5px;
    margin: 5px;
}

.wpmeetupactivity-setup-fieldset legend {
    font-weight:bold;
    padding: 3px;
    border-radius: 5px; 
    background: #aaa;
    color: #fff;
}