I've looked into Linux XDG icon packs as a source of freely licensed in-app icons optimized for multiple pixel resolutions. Tango (last updated 2009), Humanity,

I've looked into Linux XDG icon packs as a source of freely licensed in-app icons optimized for multiple pixel resolutions. Tango (last updated 2009), Humanity, and Oxygen (forgot what Gnome icons are called) are, in a way, the last of breed for multi-colored icons. Practically no modern icon themes released within the past few years have toolbar icons with outlines and colored fills. These icons have outlines. On the plus side, they look decent on any color background (though Humanity icons have too light-colored borders, which can make light icons hard to see on light backgrounds). On the minus side, the outlines look noticeably blurry when rescaled instead of picking the nearest integer size. Monochrome icons are less damaged by resizing. However, they can become essentially invisible black icons on a dark background (or vice versa), so often come in light/dark variants. Apparently KDE has an extension to XDG icon themes called "FollowsColorScheme", where the renderer will automatically recolor -symbolic icons. (Documented at https://github.com/lxqt/lxqt/issues/1181 https://github.com/lxqt/lxqt/issues/1181 )