AbsoluteTiming[
 sDefs =
  AssociationMap[
   Key["StyleDefinitions"]@*Association@*NotebookInformation,
   FrontEndExecute@
    FrontEnd`ObjectChildren[$FrontEnd]
   ];
 styleNotebooks =
  Cases[Keys@sDefs,
   Alternatives @@
    Flatten[DeleteCases[Values@sDefs, None]]
   ]
 ]

{0.069589, {NotebookObject[Defer[$FrontEnd], 243],
  NotebookObject[Defer[$FrontEnd], 83],
  NotebookObject[Defer[$FrontEnd], 220],
  NotebookObject[Defer[$FrontEnd], 215],
  NotebookObject[Defer[$FrontEnd], 105],
  NotebookObject[Defer[$FrontEnd], 4],
  NotebookObject[Defer[$FrontEnd], 5],
  NotebookObject[Defer[$FrontEnd], 6],
  NotebookObject[Defer[$FrontEnd], 75],
  NotebookObject[Defer[$FrontEnd], 81],
  NotebookObject[Defer[$FrontEnd], 82],
  NotebookObject[Defer[$FrontEnd], 104],
  NotebookObject[Defer[$FrontEnd], 106],
  NotebookObject[Defer[$FrontEnd], 107],
  NotebookObject[Defer[$FrontEnd], 186],
  NotebookObject[Defer[$FrontEnd], 229],
  NotebookObject[Defer[$FrontEnd], 230]}}