When combined with the accent colour, this allow the user a level of customization and personalization of their device and the applications they use.
As a general rule it's good to allow your application to reflect the theme and accent colout the user has selected. Sometimes, however, you may want to "force" the theme of the app. This means that regardless of the them the user selects the application always looks the same.
Look at these two versions of the same app:
It looks like all is fine.
However, if we want to support the inclusion of the `SystemTray`, so that the user can see the time, battery level, network conection status, etc. then the system theme colour will show through and there's nothing you can do about it. Apart from not include the SystemTray, of course.
If you want to use a MessageBox in your application then you'l have a similar issue. This is what the default MessageBox looks like when a light theme is selected but a dark theme is forced.
The solution is to make your own version of a MessageBox.
Creating your own version of a message box is relatively simple. Creating your own SIP/Keyboard is a lot more complicated if you want to recreate all the functionality of the built in one.
If you've built Windows Phone or Windows Store apps you can cross-promote them with AdDuplex to get more users.