Serhiy, what do you make of the TclError?  Could it be an ArchLinux-specific bug in tk?

Here is the entire function:

    def SetFontSample(self, event=None):
        fontName = self.fontName.get()
        fontWeight = tkFont.BOLD if self.fontBold.get() else tkFont.NORMAL
        newFont = (fontName, self.fontSize.get(), fontWeight)
        self.labelFontSample.config(font=newFont)
        self.textHighlightSample.configure(font=newFont)

The first 3 lines were last touched on 2014-08-03, the last 3 on 2012-10-22. The code seem unexceptional.  This works fine for me and, I presume, most everyone else.

wysaard: What tk version does Arch Linux provide you?  Check Help => About IDLE before you hit configure.