When you say the “primary” sidebar, you are referring to the one on the left? Because the left sidebar has an ID of secondary and the right sidebar has an ID of content-sidebar. And how narrow do you want to make it?
Currently, the left sidebar has an overall width of 320px (260px for the content width plus 30px of padding on each side. So you would need to adjust two values in the CSS that I gave you plus one other value for the :before value that you found previously.
Let’s say that you want to make the overall width 280px (i.e., reduce the width by 40px) and you want to leave the padding at 30px. Then you would change these rules:
@media screen and (min-width: 1080px) {
.site:before {
width: 324px;
}
}
#secondary {
width: 260px;
}
.site-content {
margin-left: 322px;
margin-right: 253px;
}
to this:
@media screen and (min-width: 1080px) {
.site:before {
width: 284px;
}
}
#secondary {
width: 220px;
}
.site-content {
margin-left: 282px;
margin-right: 253px;
}
So, to make the left sidebar 40px narrower, you would decrease the width of the .site:before, the margin-left of .site-content, and the width of #secondary by 40px.