Style stripe payment form

It's rough but it at least outlines the box for credit card details

static/src/css/app.css

@@ -107,3 +107,29 @@ div.system-message p.system-message-title {
     font-size: 12px;
     padding: 5px 0 0 12px;
 }
+
+/**
+ * The CSS shown here will not be introduced in the Quickstart guide, but shows
+ * how you can use CSS to style your Element's container.
+ */
+.StripeElement {
+  background-color: white;
+  padding: 8px 12px;
+  border-radius: 4px;
+  border: 3px solid transparent;
+  box-shadow: 1px 3px 5px 1px #e6ebf1;
+  -webkit-transition: box-shadow 150ms ease;
+  transition: box-shadow 150ms ease;
+}
+
+.StripeElement--focus {
+  box-shadow: 1px 3px 5px 1px #cfd7df;
+}
+
+.StripeElement--invalid {
+  border-color: #fa755a;
+}
+
+.StripeElement--webkit-autofill {
+  background-color: #fefde5 !important;
+}