The logo banner for English Language & Usage used to be centered. It was designed to be centered, way back when these things were created. How can I tell? Because it exhibits bilateral symmetry in a big way. The bar has matching bookend ornaments and a line motif extending from the ornaments to each side of the viewport. Even the typeface is all-caps, completing the symmetrical effect.
What we have now is not centered, and looks like a mistake:
What we should (at least) have:
You really ought to be able to see that centering is an improvement.
To make the change will be quite simple. You only need to replace one line of CSS, from justify-content: space-between;
to justify-content: center;
.site-header .site-header--container {
width: 100%;
min-height: 70px;
max-width: 1264px;
margin: 0 auto;
padding: 0 8px;
display: flex;
align-items: center;
/* justify content: space-between */
justify-content: center;
background-color: transparent;
background-image: none;
background-position: bottom right;
background-repeat: no-repeat;
background-size: auto;
}