You may further customize a theme by placing specialized graphics in the theme folder...favicons and touch-icons.
By placing an .ico format file named 'favicon.ico' into the folder, the page will be marked to be used as a favorite icon and displayed on most browser bookmarks and title bars. The image size should be 16x16.
By placing a .png format file named 'apple-touch-icon.png' and/or 'apple-touch-icon-precomposed.png' into the folder, the page will be marked to be used as an icon for mobile devices (Apple & Android). The 'precomposed' icon type will be displayed as is (according to spec) and should be 64x64 pixels.