Etherpad comes with a lot of icons on the fontawesome-etherpad font, but in case you need one that is not there yet, follow these steps:

• Go to Fontello;
• Import the existing src/static/font/config.json into Fontello (here's a how-to for this step). This will automatically select some icons on the screen:
• Click on "Download webfont";
• Unzip the downloaded file;
• On the resulting unzipped folder, copy the config.json and the four files font/fontawesome-etherpad.* into src/static/font/;
• in unzipped folder, open css/fontawesome-etherpad.css, and copy the list of icons rules like

.buttonicon-sync-alt:before { content: '\e800'; }
.buttonicon-print:before { content: '\e801'; }
...

Into src/static/css/pad/icons.css (replacing old similar css rules)

• That's it! Your code is ready for a pull request, and your icon is available for you to use. Here's an example of pull request that added an icon

Now that your icon is available, here's how to use it:

• Get icon's code from config.json;
• Use this converter to find it's HEX value;
• Place the HEX value on the CSS.