HI these settings are in the app.config.php, just copy them over to your local.config.php and change as you wish, you will need to regenerate your signatures from with in the admin panel once you have changed the settings.
# 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 want to use a custom font just download it and place it in the referenced folder.