In order to create a style-wise module/Sensor you should use the css style according to Display.
Simply add the right class to your GUI items and they will be styled in the right way!
Here's a list of available classes, buttons, panels, text fields and more.
They are all contained in the file GUI.css (this is the file running inside Display!)
You want to add a button with a label SUBMIT:
<inpunt class="protobutton" type="submit" value="SUBMIT">
with the class .protobutton the button will appear with the right style.
Have a look to all possible styles, more may come in the future, you will have them directly running in Display once they are added.
GUI.css
Also have a look to the best example STANDARD GUI ITEMS
