Add a <viewer-password-screen> element.

elements/viewer-password-screen/viewer-password-screen.css

Index: elements/viewer-password-screen/viewer-password-screen.css
diff --git a/elements/viewer-password-screen/viewer-password-screen.css b/elements/viewer-password-screen/viewer-password-screen.css
new file mode 100644
index 0000000000000000000000000000000000000000..183f439f9892150140e556aee0b0a94c84bcf188
--- /dev/null
+++ b/elements/viewer-password-screen/viewer-password-screen.css
@@ -0,0 +1,41 @@
+:host {
+  -webkit-transition: opacity 400ms ease-in-out;
+  background-color: #ccc;
+  color: #555;
+  display: table;
+  font-family: sans-serif;
+  font-size: 15px;
+  height: 100%;
+  pointer-events: none;
+  position: fixed;
+  text-align: center;
+  width: 100%;
+}
+
+#message {
+  padding-bottom: 10px;
+}
+
+.center {
+  display: table-cell;
+  vertical-align: middle;
+}
+
+.form {
+  border: 1px solid #777;
+  box-shadow: 1px 1px 1px;
+  display: inline-block;
+  padding: 10px;
+  width: 300px;
+}
+
+#successMessage {
+  display: inline-block;
+  padding-left: 5px;
+  pointer-events: none;
+}
+
+input {
+  color: #333;
+  pointer-events: all;
+}