turbolinks: Make the progress bar white and thicker

The progress bar's default color is blue. It is not noticeable as it
is similar to the blue in the page header in plinth.

Fixes #1364
Signed-off-by: Joseph Nuthalapati <njoseph@thoughtworks.com>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

static/themes/default/css/plinth.css

@@ -364,3 +364,8 @@ a.menu_link_active {
 .background-system {
     background-image: url('../img/system-background.svg');
 }
+
+.turbolinks-progress-bar {
+    height: 3px;
+    background-color: white;
+}