I'm a sucker for free themes. And dark themes. And themes which mimic the best of Android. All of which helped me make a beeline for X-theme by developers extraordinaire MMMOOO. Screenshots and link below, but you might want to be quick about grabbing the theme, since it looks like a time-limited offer?
Here's X-theme in place on my Nokia 808, it's a 1.6MB install, which is about average for a Symbian theme with a number of third party app icons:
Note that there are extra themed elements on Belle Refresh phones, including progress bars and the qwerty keyboards. You can download X-theme here, currently free as I write this.
(Is it sad that I keep reverting to Symbian themes that strongly remind me of Android elements?!)