theme: Only try to set foreground if the widget has it

This tripped up on a `ttk.Sizegrip`, so add the same `'foreground' in
widget.keys()` style of check as other code in this function.

theme.py

@@ -343,18 +343,18 @@ def _update_widget(self, widget: tk.Widget | tk.BitmapImage) -> None:  # noqa: C
 
             elif 'cursor' in widget.keys() and str(widget['cursor']) not in ['', 'arrow']:
                 # Hack - highlight widgets like HyperlinkLabel with a non-default cursor
-                if 'fg' not in attribs:
-                    widget['foreground'] = self.current['highlight']
+                if 'fg' not in attribs and 'foreground' in widget.keys():
+                    widget.configure(foreground=self.current['highlight']),
                     if 'insertbackground' in widget.keys():  # tk.Entry
                         widget['insertbackground'] = self.current['foreground']
 
-                if 'bg' not in attribs:
-                    widget['background'] = self.current['background']
+                if 'bg' not in attribs and 'background' in widget.keys():
+                    widget.configure(background=self.current['background'])
                     if 'highlightbackground' in widget.keys():  # tk.Entry
                         widget['highlightbackground'] = self.current['background']
 
-                if 'font' not in attribs:
-                    widget['font'] = self.current['font']
+                if 'font' not in attribs and 'font' in widget.keys():
+                    widget.configure(font=self.current['font'])
 
             elif 'activeforeground' in widget.keys():
                 # e.g. tk.Button, tk.Label, tk.Menu