Most Icon themes do not provide custom icons for each weather type and thus use the default app icon for each. Adding an option to use the default icon set or a set from another theme would improve the user experience with some of the popular but incomplete themes. At the moment this behaviour is consistent with downstream widgets like Daily Forecast https://store.kde.org/p/1287928 which would also benefit.
Hmm, I don't think we want to get in the business of working around buggy or incomplete icon themes. Then there's no incentive for them to get better and offer a good UX. I don't think we'll do this, sorry.
The best solution would be probably for https://www.freedesktop.org/wiki/Software/icon-theme/ to add weather icons but they most likely won't feel concerned either. I don't know if it should be counted as buggy if an icon set is missing weather icons as most "Full Icon Themes" @ https://store.kde.org/browse?cat=132&ord=rating don't provide those. Maybe for plasma at least you could use the breeze icons as a fallback instead of the basic hicolor theme.
The icon theme spec doesn't necessarily need to change for this to get better. It's an optional spec anyway, and nothing prevents icon themes from adding weather icons. > Maybe for plasma at least you could use the breeze icons as a fallback instead of the basic hicolor theme. This isn't actually a Plasma decision, it's an icon theme decision: the icon theme gets to determine what its fallback theme is. Icon themes that are targeting Plasma should definitely set Breeze as a fallback.