aerothemeplasma/Plasma/KDE Plasma Theme/Seven-Black/solid

This part of the theme is undocumented by KDE plasma and allows Dialogues and ToolTipDialogues to have custom backgrounds which are marked as "Solid".
However, nothing in KDE's code prevents you from using just about any SVG which can be either solid or transparent, as the source code merely shifts the SVG location to
this specific directory.