I'd like to start with Ruby/Tk on Linux, but the default included styles are not very good looking... I've seen screenshots of applications using Tile and looking great, so I'd like to know whether there is a collection of Tile themes somewhere (I would like something that looks like the GTK Clearlooks theme)?
I spent a lot of time googleing, but was not able to find anything...
No, there isn't a place. While Tk has had theme support for quite a while there has been virtually no community activity in creating themes outside of those that come with the standard distribution. There have been a couple of themes that people have created but they don't live in any central spot.
The main thing I'm aware of that's ongoing is Georgios Petasis's work on building themes that use GTK and Qt under the hood, and so become better able to pick up system themes on Linux. Alas, there are things missing from them for various awkward reasons AIUI. (If only the authors of those themes were better at describing the metrics of their themes, life would be even better.) I'm sure he'd welcome someone who wanted to help.
On Windows and OSX, the native theme engines already supplied do what people really want (make Tk look native) so there's little drive from that direction.