@RPNorte ,

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)

Please visit https://github.com/webkul/hotelcommerce/blob/v1.4.1/themes/hotel-reservation-theme/header.tpl#L87mfor file link.