title: Change style and position of the message box generated by withProgress()
body: <p>The <code>withProgress()</code> function can generate a message box indicating the shiny app is running. But the message is at the top-right conner of the browser with a small text size, which makes the message not that eye-catching. </p>
  
  <p>So I wonder is there any way that I could change the style and the position of this box, so that the message can be more expressive.</p>
  
  <p>Here is a part of my code:</p>
  
  <pre><code>output$plot &lt;- renderPlot({ 
   if (input$button){
  
   withProgress(
                distributionPolygon(data(),unit="years",colors=c("black","gray30","gray50","gray70","blue3","dodgerblue3","steelblue1","lightskyblue1","red4","indianred3","orange","seagreen4"),main=title(),tpsMax=input$months),
                message = 'Loading...',
                detail = ''
                )
        }
     })
  </code></pre>