Can you please check before you reply? why are you keep saying that there is no problem with the theme? I am the first who installed it and you should be thankful that I’m informing you about bugs.
Check the website in a mobile, a real mobile and you’ll see that 1st. The side page still exists on mobiles after I added the CSS code as you mentioned.
2. The theme is not 100% responsive, you can see it by yourself that the contact fields of “Contact Form 7” are not responsive.
Please go through the theme.
Screenshot from my mobile is attached with the black space on the side of the page.