Some button icons do not show up

Some button icons do not show up

I'm using jQuery mobile locally (not from CDN) and everything worked fine until now.
I want to change the icons of some buttons as described here, but some icons (e.g. bars and edit) do not work.

Edit: Tried the CDN files, which doesn't work either.

If I use for example this code:
  1. <div data-role="content">
  2.       <a href="index.html" data-role="button" data-icon="edit">My button</a>
  3. </div>

The button gets rendered with a plus icon.
The strange thing is that some icons (like plus, arrow-u, arrow-d, forward and check) work just fine.
Does someone have any suggestions what could have went wrong here?
Thanks in advance!

My folder structure is the following:
  1. ./:   index.html
  2. ./css:
  3. ./css/images: ajax-loader.gif   icons-18-white.png   icons-36-white.png   icons-18-black.png   icons-36-black.png
  4. ./js:   custom-scripting.js   jquery-1.8.3.min.js   jquery-1.8.3.js

The head of my html file looks like this:
  1. <head>
  2.     <meta charset="utf-8">
  3.     <meta name="viewport" content="width=device-width, initial-scale=1">
  4.     <link rel="stylesheet" href="./css/" />
  5.     <script src="./js/jquery-1.8.3.js"></script>
  6.     <script src="./js/custom-scripting.js"></script>
  7.     <script src="./js/"></script>
  8. </head>