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You can use the following CSS to change the spacing around the post content:
.entry, .excerpt {
padding: 2.25em 13.333%;
}
You can copy and paste that code into the “Custom CSS” section in the Customizer (Appearance > Customize). Once added there, it should take affect right away.
The code above is going to leave the spacing the same initially. You’ll want to change the “13.333%” value to increase or decrease the spacing on the left and right of the post content.
Changing the space around the post container (the gray area) is a bit more complicated, but can be changed like this:
.main {
margin-right: 3.703%;
margin-left: 7.407%;
}
You reduce the margin-left value to move the content further left, and vice-versa.