im not sure if i understand u correctly.. but i give it a try:
If you want to change the blog width to 100%, for example on this page:

https://focus-om.com/2017/11/09/visioning-prosperity/

You need to change the max-width, size for your .container class.
As far as i see, you have different Media Queries (for Desktop View, Mobile View etc..)

So, if you only want to change the look fullwidth on your desktop view, edit in the following media query:

@media screen and (min-width: 960px){
/* ... */ 

.container {
    //max-width: 960px; /* old value */
    max-width:100%;
}

/* ... */ 
}

But beware, you change the width size on all pages. If you only want to affect the single blog post sites, you could expand and overwrite your code.
For example like this:

@media screen and (min-width: 960px){
/* ... */ 

.single-post .container {
    //max-width: 960px; /* old value */
    max-width:100%;
}

/* ... */ 
}

Hope that could help a bit.