Try removing memo's background on applying style event.
procedure TForm1.Memo1ApplyStyleLookup(Sender: TObject);
var
BckObject: TFmxObject;
begin
BckObject := Memo1.FindStyleResource('background');
if Assigned(BckObject) and (BckObject is TSubImage) then
begin
TSubImage(BckObject).Source := nil;
end;
end;