[matrix] Style "View Documentation For" dropdown

docs/_static/style.css

@@ -300,6 +300,24 @@ header > nav > a:hover {
 
 .sub-header > select {
   display: none;
+  background-color: transparent;
+  padding: 0.4em 0;
+  font-size: 1em;
+  width: 13.5%;
+  color: white;
+  border: none;
+  border-bottom: 1px solid var(--search-border);
+
+  appearance: none;
+  background-image: url('/_images/drop_down_icon.svg');
+  background-repeat: no-repeat;
+  background-position-x: 100%;
+  background-position-y: 50%;
+
+  cursor: pointer;
+}
+.sub-header > select:focus {
+  outline: none;
 }
 
 .sub-header > .settings {

docs/images/drop_down_icon.svg

@@ -0,0 +1,4 @@
+<svg fill='white' height='24' viewBox='0 0 24 24' width='24' xmlns='http://www.w3.org/2000/svg'>
+    <path d='M7 10l5 5 5-5z'/>
+    <path d='M0 0h24v24H0z' fill='none'/>
+</svg>