Creating Post Gaps via Border Bottom

Im trying to seperate each post from one another insstead of just a ——————- type line. Here is what I added.

.post{
    margin-bottom: 20px;
    padding-bottom: 20px;
    border-bottom: 35px solid #e4e4e4;

#e4e4e4 is the color of my themes background

The problem is the border bottom line or gap I made doesnt extend all the way to the sides. How can I get it to extend all the way?