Not sure if this is the clean way of doing but this is my suggestion:
These icons are rendered using fonts rather than images. Allowing you to resize/ colour and play with many css techniques. Each icon has a code eg: 'e\805', 'e\804' etc..
1. Open http://fontello.com/ and select the icons you wish to have and download the font files. (you can customise the codes on this website for the icons you wish to be included in your font files).
2. Once downloaded. Replace the codes appropriately in the theme css file and also replace the font file with new one under font directory.
A slightly better way would be to download and create a new font-family using the above approach. and using this new family in css as above. This way, any old stuff wont be affected.