The translation file en_US.pot you can find in Languages_PO_files folder in theme package. Edit the .po file using POEdit, use the translation field to make replacements. Read here how to do it right.
From the file menu, save file with your language name e.g de_DE.po. It will generate both a .po and .mo files for your translation. Place these files into languages folder /wp-content/themes/lightbook/languages.
Next edit wp-config.php located in the root folder of WordPress and use the $locale with prefix with your language name which should be similar to the translation file name. For example: $locale=’de_DE’;