Website not responsive on smartphones

Hi,
can you please help me. i have the premium theme of fifteen theme and it works fine on laptops but i can only view half of all my pages on my iphone.
the website is https://www.arslondon.com.
Also, i would like to change the colour of the “transparent bit”on the home page,where posts are dispalyed from grey to another colour. is this possible?
thank you