The problem I find is connected to the rendering of the transparency of the fill defined in the theme. Used as a theme on its own, contours is rendered with spurious segments of contours when the zoom level is changed.
However, it seems to work as an overlay:
1. Select a non-vector map as the base
2. Select a vector map as the overlay
3. Blend=disabled, etc
4. Select contours as the theme
5. Zoom in and out.
As an alternative, set the background fill in the contours theme to #FFFFFF, and the blend to multiply in the overlay. That way the theme works as a standalone map and as an overlay.
I attach a slightly simplified theme as well as an opaque version.