The plugin uses his own theme directory so changing the weather icons is only possible by replacing them. I wrote a quick&dirty patch who changes this. The icons will be loadet via the xfce_themed_icon_load method and the filename was changed to xfce4-weather-XY (from: XY) P.S. Sorry for my bad english :)
Created attachment 2988 weather-themable.patch
Created attachment 2989 weather-themable2.patch I played a bit and here is a new patch: It does the same as bevore but fixes the Makefile(s) and the icon-names. I can use this patch in the build process of gentoo linux and it works. :)
Created attachment 2992 new-weather-themable.patch Patch rewritten (less changes in panel-plugin/weather-icon.c)
Created attachment 2993 weather-themable-rev4.patch Even more simplyfied (use g_strdup_printf and not strcpy/strcat)
Created attachment 2996 weather-themable-rev5.patch fixed an error in the configure script
Long time ago. Don't know how much of it is still usable after all the changes, and what's already in git. Let's look into it after the next release.
With http://git.xfce.org/panel-plugins/xfce4-weather-plugin/commit/?id=01787dffe9470ef806d6b0f5089127de316ef5ed, the plugin finally supports loading a custom theme (but not fdo-standardized themes, and probably never will) from an arbitrary directory. Automatic theme detection and GUI elements for easy theme selection are still missing but planned for the new configuration dialog which will be available in upcoming 0.8.3. I'm closing this bug now before it starts to rot ;-)