Moving "add to cart" button to quantity line on mobile

Site URL: https://islaskincare.com/products-test/p/dynamic-duo

Hi there,

I've managed to move the "add to cart" button to the same line as "quantity". However, once I go onto mobile view, the "add to cart" button gets pushed underneath the quantity button as its own line. Is there any way to fix this so that it's all one line? Incase you need a password it would be "password321". 

Also, I've tried all sorts of code to make the font size smaller on the product description but nothing seems to work. Any idea on how to fix this also would be wonderful. 

Thank you so much!