Rather than try to explain the problem, let me describe how to duplicate it:

First, make sure you have the weather applet in your panel. Go to Customize Theme and set window border to Ambiance. Set Icons to Humanity-Dark. Notice the decidedly non-monochrome-looking icon for the weather applet. Now go to Controls and toggle between Ambiance and Dust. Watch the weather applet icon, as well as the other panel icons. Weather applet icon changes to something similar to ubuntu-mono, but not exactly (it's monochromatic even though icon style is set to humanity-dark).

Any ideas on how to keep the weather applet icon from changing when applying Ambiance Controls? I didn't think the Controls had any relationship to the icons.

Thanks in advance.

Mods, feel free to delete similar post in Art and Design forum. I don't think that was the best forum to post this question. I asked there, but got no replies.