Not sure if this is a Discourse bug, or a site theme issue, but I have seen it discussed before (https://meta.discourse.org/t/topic-admin-wrench-is-covered-by-customized-header-bar/16721).
On a Discourse I moderate the moderator wrench is partially covered by the header when the page is scrolled all the way to the top.
If works perfectly finem but it still seems to be a UI error. Scrolling the page at all will cause the wrench to be uncovered.