The jquery-ui.structure.css file provides the structural CSS rules
(such as margins and positioning), and the jquery-ui.theme.css file
contains the theming rules (such as backgrounds, colors, and fonts).
This setup allows you to easily change themes by swapping in new
jquery-ui.theme.css files. You can also build a theme from scratch by
building on top of jquery-ui.structure.css. The jquery-ui.css file
contains both jquery-ui.structure.css and jquery-ui.theme.css.
I don't get the point of this, so basically `jquery-ui.css` =
`jquery-ui.structure.css` + `jquery-ui.theme.css`, is it now
recommended to include two CSS files instead of one for this
modularity? I don't understand the reason why the jQuery UI team
decided to do this.