This morning I had a discussion with Robin van het Hof and Vladimirs Semikins on Twitter (storify link) about custom UI5 themes. Vladimirs indicated that he created a theme of approximately 3,000 lines of CSS. Although probably quite some of those lines are generated using LESS (I hope), I have the feeling that he made quite an effort into turning his theme into something really custom.
Of course I was very interested about what it eventually looked like and asked him if he could share a few screen shots. He will have to go back to ask for permission, but I'm hoping for the best.
At the same time, Robin mentioned that he is going to use UI5 for the external marketing site of his company as well, promising another excellent custom UI5 theme to see the light. He explicitly mentioned that he would make an effort to NOT make it look like UI5.
I think it would be nice and fun if we could bundle our UI5 theming efforts somehow and create a showcase of screenshots of custom UI5 apps or sites, to form an inspiration or perhaps even boilerplate for others. Because of that, I've set up a Wiki page for this purpose on Custom UI5 themes showcase. I hope many UI5 theme developers will show what they have made there.
To kick it off, I have added some screenshots of an OpenUI5 application I have built for Ziggo. It would be great if you could also add your screenshots to the Wiki page. Adding your screenshots is already super-duper, adding a zip with the theme will instantly turn you into a real UI5 theme hero, and will grant you eternal honour and everything 
Click here to add your theme --> Custom UI5 themes showcase