The css of black black background is written in the header file itself.
You have to open /themes/hotel-reservation-theme/header.tpl and at line 87 change the background-color code as per your desire. (Current color code is #252525)
The background color can only be changed from the code. We have not provided an option to change the color of the background.
So to change the color you will have to change the code.
And if you need to have our help in that then please raise a ticket at https://webkul.uvdesk.com/en/customer/create-ticket/