There is a file in your theme called "_like_page.html" that is used to display those buttons on each page. The standard share buttons in there come directly from Facebook and Twitter and cannot be modified, but if you are experienced in editing HTML and CSS, you could remove them and add in some icons of your own, linking them to the appropriate share URLs. Here's more info on that: http://www.kodingmadesimple.com/2014/02/how-to-create-round-social-media-icon.html