Black background in admin calendar

As you can see on screen attached below (or here: https://wrzucacz.studiowizjo.pl/uploads/studiowizjo/p1be0cak4f1cdhseq8edi4a1ih34.jpg) the 1st May background is black while Im editing one of my calendar. I tried to override styles by putting below code:

.status-10 {
    background-color: #BA7C1B !important;
}

in files:

• wp-content/plugins/wp-simple-booking-calendar-premium/css/wpsbc-calendar.css
• wp-content/plugins/wp-simple-booking-calendar-premium/css/wpsbc-admin.css

but it doesnt help. I cannot even figure out in Chrome inspector what rule is causing that and override it anyway.

I tried also to change theme to Twenty Eleven, but it doesnt make any change.