code: def simple_polygon_without_brush(layer, width='0.26', color=QColor('black')):
      
      registry = QgsApplication.symbolLayerRegistry()
      line_metadata = registry.symbolLayerMetadata("SimpleLine")
      symbol = QgsSymbol.defaultSymbol(layer.geometryType())
  
      # Line layer
      line_layer = line_metadata.createSymbolLayer(
          {
              'width': width,
              'color': color.name(),
              'offset': '0',
              'penstyle': 'solid',
              'use_custom_dash': '0',
              'joinstyle': 'bevel',
              'capstyle': 'square'
          })
  
      # Replace the default layer with our custom line
      symbol.deleteSymbolLayer(0)
      symbol.appendSymbolLayer(line_layer)
  
      renderer = QgsSingleSymbolRenderer(symbol)
      layer.setRenderer(renderer)
docs: Simple style to apply a border line only to a polygon layer.
  
      :param layer: The layer to style.
      :type layer: QgsVectorLayer
  
      :param color: Color to use for the line. Default to black.
      :type color: QColor
  
      :param width: Width to use for the line. Default to '0.26'.
      :type width: str