I noticed that majority of themes use nested raster images insinde svg for widget shadows. I tried to add large shadow (like in the new air style) for widgets but using vector element. It does not look quite right in plasma. Is there some sort of bug here?

If I recall correctly, Plasma Desktop depends upon the SVG rendering capabilities of Qt, using the QtSVG module. The QtSVG renderer is unfortunately weak in certain areas - you have likely run into a bug with it - which is why others include rendered raster images instead.

Yes, I think so.

Hopefully this can be solved some day.

After looking trough different themes I start to think that this is not quite related to the shadow itself but rather to the mask. After removing mask my shadows look perfectly fine.

Can someone explain how to use mask in KDE plasma themes correctly, please?

Interesting. As Plasma code itself is not responsible for rendering the SVG files, the inability to use masks is still likely a bug in QtSVG i'm afraid. It does give more information on where to look for the bug though - which may be useful if someone is interested in attempting to fix it.

You could try asking the other theme developers, or the Plasma developers themselves, if anyone has managed to use masks in a SVG file successfully before.

Actually masks works as intended. It is just me, looking at wrong themes!

After taking carefull look at air theme and reading about svg standard I was finally to make everything what I wanted to do from the very begining.

If you have doubts about plasma themeing just model after Air theme.