nOptionMenu targetlist contains a readable description of the
        tags applied to Python source within IDLE.  Selecting one of the
        tags from this list populates highlight_target, which has a callback
        function set_highlight_target().

        Text widget highlight_sample displays a block of text (which is
        mock Python code) in which is embedded the defined tags and reflects
        the color attributes of the current theme and changes for those tags.
        Mouse button 1 allows for selection of a tag and updates
        highlight_target with that tag value.

        Note: The font in highlight_sample is set through the config in
        the fonts tab.

        In other words, a tag can be selected either from targetlist or
        by clicking on the sample text within highlight_sample.  The
        plane (foreground/background) is selected via the radiobutton.
        Together, these two (tag and plane) control what color is
        shown in set_color_sample() for the current theme.  Button set_color
        invokes get_color() which displays a ColorChooser to change the
        color for the selected tag/plane.  If a new color is picked,
        it will be saved to changes and the highlight_sample and
        frame background will be updated.

        Tk Variables:
            color: Color of selected target.
            builtin_name: Menu variable for built-in theme.
            custom_name: Menu variable for custom theme.
            fg_bg_toggle: Toggle for foreground/background color.
                Note: this has no callback.
            theme_source: Selector for built-in or custom theme.
            highlight_target: Menu variable for the highlight tag target.

        Instance Data Attributes:
            theme_elements: Dictionary of tags for text highlighting.
                The key is the display name and the value is a tuple of
                (tag name, display sort order).

        Methods [attachment]:
            load_theme_cfg: Load current highlight colors.
            get_color: Invoke colorchooser [button_set_color].
            set_color_sample_binding: Call set_color_sample [fg_bg_toggle].
            set_highlight_target: set fg_bg_toggle, set_color_sample().
            set_color_sample: Set frame background to target.
            on_new_color_set: Set new color and add option.
            paint_theme_sample: Recolor sample.
            get_new_theme_name: Get from popup.
            create_new: Combine theme with changes and save.
            save_as_new_theme: Save [button_save_custom].
            set_theme_type: Command for [theme_source].
            delete_custom: Activate default [button_delete_custom].
            save_new: Save to userCfg['theme'] (is function).

        Widgets of highlights page frame:  (*) widgets bound to self
            frame_custom: LabelFrame
                (*)highlight_sample: Text
                (*)frame_color_set: Frame
                    (*)button_set_color: Button
                    (*)targetlist: DynOptionMenu - highlight_target
                frame_fg_bg_toggle: Frame
                    (*)fg_on: Radiobutton - fg_bg_toggle
                    (*)bg_on: Radiobutton - fg_bg_toggle
                (*)button_save_custom: Button
            frame_theme: LabelFrame
                theme_type_title: Label
                (*)builtin_theme_on: Radiobutton - theme_source
                (*)custom_theme_on: Radiobutton - theme_source
                (*)builtinlist: DynOptionMenu - builtin_name
                (*)customlist: DynOptionMenu - custom_name
                (*)button_delete_custom: Button
                (*)theme_message: Label
        z