How to change the symbol of the extra menu?

To resolve this issue, below is our instruction:
Firstly, follow the jetpack’s guideline in the Blog article on how to use Edit CSS here. http://designwall.com/blog/using-jetpack-custom-css-for-your-wordpress-site/
Then add the code to the Edit Css. (E.g: use icon size 40x40px)

.select-page .dropdown-toggle span { 
      background: url('FILE URL');
      border: none; 
      height: 40px;
      width: 40px;   
}
.select-page .dropdown-toggle {
     border: none;
}

Note: Please add your icon image to the Media then copy FIle URL on the right hand side and change here: FILE URL