hugolib/filesystems: Fix theme config for Work Fs

Which was the reason why `PostCSS´ from theme(s) was not picked up.

Fixes #4951

hugolib/filesystems/basefs.go

@@ -407,6 +407,11 @@ func (b *sourceFilesystemsBuilder) createFs(
 	s := &SourceFilesystem{
 		SourceFs: b.p.Fs.Source,
 	}
+
+	if themeFolder == "" {
+		themeFolder = filePathSeparator
+	}
+
 	var dir string
 	if dirKey != "" {
 		dir = b.p.Cfg.GetString(dirKey)