Fix support button and ToC sidebar from appearing on top of zoomed-in images (#1137)

* Make zoomed-in images overlay support button and ToC

* Make announcement bar overlay zoomed-in images

Author: josh-wong

src/css/custom.css

@@ -62,6 +62,7 @@
 div[class^='announcementBar'] {
   font-size: 1.1rem;
   padding: 20px 0;
+  z-index: 21; /* Ensures the announcement bar stays visible when images are zoomed in on */
 }
 
 /* Home page cards */
@@ -360,3 +361,13 @@ hr {
 html[data-theme="dark"] .container img[src$=".png"] { /* Adds a white background to transparent .png images. */
   background-color: #ffffff;
 }
+
+.medium-zoom-overlay {
+  z-index: 15;
+  position: fixed;
+}
+
+.medium-zoom-image {
+  z-index: 20;
+  position: relative;
+}