Construct a vector of styles related to 'fill'
     
    fill_style(state, geom)
    Arguments
    
    
    
      | state | 
      list including 'gc' (graphics context)  | 
    
    
      | geom | 
      which geometry has asked for a style  | 
    
    
    Value
    character vector of multiple "attr_name: value" strings for 'fill',
        'fill-opacity'