Set #header height: 18px, which was already the natural height.

No visual change now, but writing it down helps communicate intention
with regards to the size of other elements.

frontends/etiquette_flask/static/css/common.css

@@ -82,6 +82,7 @@ pre
     display: flex;
     flex-direction: row;
 
+    height: 18px;
     margin: 8px;
     background-color: var(--color_transparency);
 }