Plugin that loads a custom CSS to affect the user interface.
This plugin uses the WebappStaticResourcesFolder plugin extension to expose the web/static folder at
http://example.com/oxygen-xml-web-author/plugin-resources/custom-ui-style-plugin/.
The plugin.js file adds a link html element to the head of the Document
to load the web/static/style.css file in the Web Author HTML page.
The style.css file changes the color of the header-bar at the top of the page. Add your own custom CSS here as needed.