Hi designers, thanks a lot for Breeze, it is really an amazing theme and icon set!

In Breeze 5.4 there are a lot of icons for several applications. In web-world I saw icons for Firefox and Chromium, among others. What do you think about an icon to Thunderbird? That application is utilised for a lot of people, so it would be nice to have a breeze icon for that! =)

Thank you again;

Hi artists, just in case someone forgot this topic and because Plasma 5.5 is going to freeze in less than one month, I am writing a ping here.
Thank you for all the amazing work.

Two years now, and we don't have any changes with it. How I can help?

I suppose that depends on how you are able to help.
The github mirror for the breeze icon theme is https://github.com/KDE/breeze-icons
Instructions are available there for how to get involved and contribute.