I would like to have a way to get the theme colour of the users Windows theme with WPF or C# code-behind. And with this colour I mean the one that can be set in Personal
I think DwmGetColorizationColor (http://msdn.microsoft.com/en-us/library/aa969513.aspx) is what you're looking for; you'll need to P/Invoke.
DwmGetColorizationColor
The theme you see on Windows is based on Glass. You have to use a glass in your application to get the same effect.