In APEX 5.0 theme 42 is included - this is the Universal Theme. The theme comes with two styles: Blue and Gray.
The UI is so much improved in APEX 5, it's really a game changer. When you look at the details for example from such a style you will see it's using a Less file (Less is a CSS pre-processor) behind the scenes. So it's very easy to make changes to the CSS. But you probably even don't need to do that as the development team allows you to make customisations with ThemeRoller which generates JSON (another nice example where JSON is used - interested in JSON - I did some other posts on this blog about JSON).
To change the color for example, below you see the JSON for that.
Note that Theme Roller will generate the JSON for you. Accept from those customisations, the blue and gray style are using the exact same css.
From the Blue style many screenshots have been posted already, but I love the Gray style too, here's a screenshot when you switch the sample application's style to Gray.