[oldlaptop]

The theming engine is absolutely "built-in", as are a handful of themes; in particular aqua and the winnative/xpnative/vistanative family (which integrate with the Mac/Windows windowing system such that the application will look "native" by default on those systems).

[bachmeier]

> which integrate with the Mac/Windows windowing system such that the application will look "native" by default on those systems

That's my point. You can make an app that looks okay on those OSes, but all of the built-in options for Linux look ridiculous, and then you're back to the third-party stuff.

[oldlaptop]

I don't think clam looks "ridiculous", but I suppose we're in to subjective stuff now. :)

In principle this isn't really any different from GTK or Qt theming (where it's up to users to go forth and find themes they like, and set the default as desired), but in those cases you tend to get themes packaged up by distributions and GUI configuration support from desktop environments, and neither seems likely to happen for Tk.