fix problem with banner not being centered on keyboard navigation

misc/system-wide.css

@@ -1,6 +1,6 @@
 <style>
 #bannercontainer {
-        width: %100;
+        width: 100%;
         display: flex;
         flex-direction: row;
         flex-wrap: wrap;
@@ -21,6 +21,13 @@
         }
 }
 
+/* this code makes the <a> tag that holds bannercontainer a block element,
+so that it can take up the full width of the screen */
+a:has(#bannercontainer) {
+    display: block;
+    width: 100%;
+}
+
 /* tighten up elements at the top of the AtoZ */
 #s-lib-public-header-desc { margin-top: -30px; }
 #s-lib-public-main-searchbar { margin-top: -20px; }