Can we use different theme same time?

For example

Main window is dark theme Dialog is light theme

is it possible?

I guess you could if you use different different QmlEngine for each component. But I think it will require extra work if you want that inside the same QmlEngine. The library have been designed with a singleton approach in mind.

If only a small part should be light, I would advise you to create your owned components for with your colors.