Use accent color as the default color for controls

**Related to https://github.com/microsoft/ProjectReunion/issues/232**

Today WPF uses Blue as the default colour instead of the user's accent colour. This is the same for WinForms too but since it's GDI based it can be tweaked easily but WPF controls are rendered independently so it needs internal modification for WPF. Please start using the accent colour on Windows 10 for a consistent user experience.