[oldlaptop]

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.

[bmacho]

Those are TK? They look good!

[bachmeier]

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.

[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).