Yes it is possible, its in the app config.php, Copy it to your local.config.php and edit as you wish
# Options for the signature that's generated
Config::Set('SIGNATURE_TEXT_COLOR', '#000');
Config::Set('SIGNATURE_USE_CUSTOM_FONT', true);
Config::Set('SIGNATURE_FONT_PATH', SITE_ROOT.'/lib/fonts/tahoma.ttf');
Config::Set('SIGNATURE_FONT_SIZE', '10');
Config::Set('SIGNATURE_X_OFFSET', '10');
Config::Set('SIGNATURE_Y_OFFSET', '17');
Config::Set('SIGNATURE_FONT_PADDING', 4);
Config::Set('SIGNATURE_SHOW_EARNINGS', true);
Config::Set('SIGNATURE_SHOW_RANK_IMAGE', true);
Config::Set('SIGNATURE_SHOW_COPYRIGHT', true);
If you wish to use a custom font then make sure you download the font from a site and upload it to the dir specified