Hi,
is there a way to make MarbleWidget transparent (i.e. partially showing what's under it?).

That was possible at one point some years ago (IIRC by setting the Qt::WA_NoSystemBackground Attribute). But so far during the last seven years of Marble development nobody has really used or requested this feature. Therefore I'm I'm afraid this feature had been removed to keep the code simple and performant.

I might be implementing transparency for MarbleWidget - do you have any tips on how to approach it?
I can see that (in TextureLayer) Marble is using QRgb format for QImage and a QPainter with no set opacity. Should it work when I modify these aspects, or is there something else I should be aware of?

Check for the places where the attribute above is mentioned.