Custom CSS Mobile Responsiveness

I added custom CSS to my site’s blog posts. Primarily changing the margin for the post description, so it was centered. I also changed the width and height of the featured images for blog posts.

The only issue is that for mobile, the pages are looking a bit strange. The blog images are cut off and when it comes to the paragraphs, after each word there is a break, making the page extremely long. A good page would be to compare this blog post of ours on desktop vs. mobile: http://aquilasw.com/2018/01/10/promoting-careful-not-kill-top-performers/

Can you help me keep my custom CSS in place while making the pages mobile responsive?