NumericUpDown: Default foreground doesn't always match theme

[ionsharp]

If using the dark theme, the foreground is black when not in focus. It should be white like TextBox and then black when in focus. The only way to do this as is would be to explicitly define a style and set the foreground to the appropriate ThemeResource so it displays correctly for all themes.

[ionsharp]

The best ThemeResource to use for this is SystemBaseHighColor because none of the predefined resources for TextBox have ideal values. The closest would be TextBoxBorderThemeBrush, which is #FFFFFFFF for dark and #A3000000 for light.

[xyzzer]

Please send a pull request and I'll integrate it after a review and test.