The #1 support request I get about WinterBoard is confusion over the priority order of wallpaper. The issue is that Wallpaper.html files always render over .png files, even if they are in a lower priority theme. This means that any theme with a fully opaque Wallpaper.html (most of them, as they are the wallpaper) breaks the priority system.
To fix this, I changed WinterBoard to understand "this theme has wallpaper" and fully conflict them. This really makes a lot more sense when you are using 99% of the themes out there. However, this breaks themes that provide HTML overlays that are designed to work with other themes that have PNG files. So far I only know of a single example: the weather widget, which I believe comes in WbApp.
The solution I have come up with for this is that WinterBoard now supports "Widget.html". This is much more flexible and much more awesome: all Widget.html files are rendered, in priority order, above the wallpaper. This means that multiple things like Weather can be installed at once. For an example of how this works, WinterBoard now comes with "Dim Wallpaper", a theme which uses it to provide a wallpaper dimming layer.
This functionality comes as part of version 0.9.2600-1. You may want to Depends: >= it in such packages. Otherwise, the fix for all such "theme" packages is to just rename Wallpaper.html to Widget.html.