I found the reason, the theme files doesn't paint the background.
The background is a .png file referred by the svg files. When
you select a theme for the clock, I copy the theme files to a
configuration directory
(/home/user/.desktop_clock_widget/theme/"NUMBER-OF-THIS-WIDGET")
But I do not copy this .png file.

You can try this:
Find out which theme directory in the config directory belongs
to this widget (for example:
/home/user/.desktop_clock_widget/theme/desktop-clock-widget.desktop-0)
And copy the Clock.png and Clockmiddle.png to this folder.

Nicolai