move modal dialog custom styles to its own file

src/app/ui/modal-dialog-custom.js

 var modal = require('./modaldialog.js')
 var yo = require('yo-yo')
-var csjs = require('csjs-inject')
+var css = require('./styles/modal-dialog-custom-styles')
 
-var css = csjs`
-  .prompt_text {
-    width: 300px;
-  }
-`
 module.exports = {
   alert: function (text) {
     modal('', yo`<div>${text}</div>`, null, { label: null })

src/app/ui/styles/modal-dialog-custom-styles.js

+var csjs = require('csjs-inject')
+
+var css = csjs`
+  .prompt_text {
+    width: 300px;
+  }
+`
+
+module.exports = css