Since version 8.5 Tk ships with themed widgets that by default try to look native on Windows and OS X by leveraging the respective platforms' native GUI toolkits for drawing.
Wx tries, but IMO fails to varying degrees to feel "right". WxGTK in Linux is okay-ish (everyone expects shitty UX from GTK programs anyway); WxMSW is an eye-sore. I haven't dared using Wx under OSX yet.
For the details see https://tcl.wiki/14796.