Firstly you need to select the box width to make visible to background, so you can follow the following steps for it;
Go to Dashboard => Appearance => Customize => Basic Setting => Then select the page width to 1200px or 1500px => And save it.
After that you need to remove the header background image or you can replace the image, if you remove the image then you will be able to keep the background color, so now please follow the following steps to remove the background image;
Go to Dashboard => Appearance => Customize => Background Image => Click in remove => Then save it.
And at last if you want to keep the background color in site, then please follow the following steps to fix your issue:
Go to Dashboard => Appearance => Customize => Colours => Body Background => Change it and click in save.
Please let us know if we are understanding you fully or if perhaps this is not what you meant.