Sorry to hear you are having issues. But, as a user, when I apply "Default Theme" is just like saying "disable Windowblinds", no? Hence Windowblinds can't control the double click on taskbar action any longer.
Stardock Community Assistant.
Basj would be correct. But, if I am not mistaken, you can right click the titlebar and choose rollup this window.
In the case that I am wrong there, it must be a WindowFX function.