The Plover Icon
Some icons were taken direction from the MIT-licensed icons8/flat-color-icons. Icons8 were very helpful to me and my support request and I recommend looking into their non-free offerings as well. All of the icons I use from them are from the free repository, though.
I’ve also taken and modified some icons from google/material-design-icons. They needed to be colored and spruced up but Google provides great variety in their free set.
There are some GPLv3-licensed icons that provide good variety and detail that I’ve simplified for Plover. linuxdeepin/deepin-icon-theme
The font selector icon was created with the Google Web font Averia Libre