The Plataro SVG icon theme, by Pedram Pourang (aka Tsu Jan tsujan2000@gmail.com), consists of flat and colorful icons intended for use under KDE and LXQt. Although it is a rich icon set, it takes its missing icons from Breeze, Oxygen or GNOME icon theme, whichever is installed and in that order.
Plataro was made based on FaenzaFlattr, when the latter disappeared from Internet, but many of its icons are made based on nouveKDEGray, Oxygen and GNOME's default icon set, with emphasis on clarity, colorfulness and flatness.
"Plataro" means "a set of flat things" in Esperanto.
With Qt apps, for the symbolic monochrome icons to have a hight contrast with dark backgrounds, a good Qt plugin is needed. A standard installation of KDE or LXQt provides such a plugin.