Update src/JeffWilcox.PhoneThemeManager/ThemeManager.cs

There's a very minor issue when run on WP8 where some colors don't match the default themes. I think it's because some resources use different naming conventions than in WP7 (particularly PhoneRadioCheckBoxPressedBrush and PhoneTextBoxEditBorderBrush which both use PhoneAccentColor as the color) causing ThemeManager to overwrite them with wp7 colors rather than WP8 ones.

My fix adds a check for version and only adds the two items above if on WP7 (or leave them as accent color if on WP8)