docs: def css_style_apply():
      """
      -----
      Brief
      -----
      Function that apply the css configurations to the Jupyter Notebook pages.
  
      -----------
      Description
      -----------
      Taking into consideration that, ultimately, biosignalsnotebooks Jupyter Notebooks are web pages,
      then a CSS file with the style options for customization of biosignalsnotebooks environment become
      particularly important.
  
      With CSS, the Jupyter Notebook template can be adapted in order to acquire the opensignals
      format, by changing background color, color pallet, font style...
  
      This function simply injects an HTML statement containing the style configurations.
  
      Returns
      -------
      out : IPython.core.display.HTML
          An IPython object that contains style information to be applied in the web page.
  
      """
  
      path = "styles/theme_style.css"
      if pathexist(path):
          style = open(path, "r").read()
      else:
          style = open("../../" + path, "r").read()
  
      print(".................... CSS Style Applied to Jupyter Notebook .........................")
      return HTML("<div id='style_import'>" + style + "</div>")