[escoz]

Themes don't work; Windows Phone has two choices and a lot of developers simply support only of them.

[nigelsampson]

Yes and No :-)

Most utility apps that aren't supporting an existing brand will often support both themes. However I've noticed that apps that are already branded will go along with only the theme that works best with their brand.

[mixmastamyk]

The way themes have worked historically is that the application isn't consulted.