Color and opacity of Menu bar background

Hello,
so i don’t know what happend, but on front page (with slider) menu bar has lost background, it’s fully transparent. I want to bring back semi transparent black bacground. There is some CSS code to do that?

Hi,

Put the code below to: customize > additional CSS

.site-header {
    background-color: rgba(0,0,0,0.5);
}

Regards,
Awan