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)
Currently, we made the hero image to acquire the full screen. No matter what the size of the screen is the image will adjust itself to take the full screen.
That is why we cannot make it responsive.
And we have taken the best possible steps to manage the hero image.
The slider is another topic. Although we have a QloApps Slider addon it does not make the hero image a slider.
We have it on our roadmap and will take the necessary action soon.