A theming engine went in something like 15 years ago now; the default theme looks rather dated, but there are plenty of others. See https://wiki.tcl-lang.org/page/List+of+ttk+Themes (though the screenshots of core themes are from 8.5/8.6 - default in particular has changed a bit in Tk 9).
The "catch" is that the theming engine has its own new widgets, and so to be themed an application has to use the new API. Code from 1995 (or 2005) still produces GUIs from 1995.
Yeah, they added that a long time ago, but it's not at all the same as built-in support. My experience was trying out fragile third-party projects with little or no documentation. The whole point of using Tcl/Tk for writing a GUI was the convenience.
Things may have changed over time. I no longer pay attention. Maybe I'll give it another try.
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).
> 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.
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.
The "catch" is that the theming engine has its own new widgets, and so to be themed an application has to use the new API. Code from 1995 (or 2005) still produces GUIs from 1995.