First we look in the sourcode and see that the checkbox is a ThemedWidgetInterface and it's an SVGDoes anyone know, where KDE folk maintain Plasma's visual assets like checkbox or radio buttons, that look like that. I've file-searched quite some repositories in github.com/KDE for svg and png, but no luck so far. Or, do they maintain, in some obscure repo, some Brainfuck code, that produces the graphics on demand?
Then we use tineye to reverse search your button and it's KDE Breeze <-- that's the GIT repo for Breeze
Now, the code in that kde forum page already reveals it is generated in the QT language, rather than an SVG.
note: This was during the development stage of Breeze, they used QT markup language as RAD to define it.
If you search for checkbox in the code:
so that's where I stranded too..