#Technically, it's bydesign.
*But really, it's just godawfuldesign.*

The bright borders are part of the background:

![enter image description here][1]

The dark borders, on the other hand, are defined in the CSS:

    .nav li {
        border-left: 1px solid rgba(160, 50, 10, 0.3);
        float: left;
        margin: 0;
        padding: 0;
    }

These don't align properly (yay boxmodel, random changes and stuff like that) and as such, this bug occurs.

  [1]: https://i.sstatic.net/otycg.png