Happy to report that Windows App Store version of the XLStyles Tool app successfully tested on the latest version of Windows 10 technical preview. The builds before 9840 had an issue in full screen mode: file picker would remain hidden (new “windowed” mode worked just fine). Latest update has a fix for that. The issue wasn’t XLStyles Tool specific. File picker was hidden for all modern apps (e.g., Adobe Reader) when launched in full screen mode which I personally prefer for modern apps.
UPDATE [11/25/14]: Windows 10 build 9879 has new file picker dialog for modern apps that run windowed desktop mode. This change breaks all modern apps that use file picker when the user clicks cancel (i.e., user decided not to pick a file). If it doesn’t get fixed by RTM I may have to publish an update. Please report any other issues if you are using Windows 10 Technical Preview.