by Sakshi Goel.
Hello Syed ,
In Moodle 2.7 all themes such as brick, standard, afterburner, boxxie, formal white and many more except a few such as clean, more are removed from the Moodle core. Now you can download these themes from Moodle.org https://moodle.org/plugins/browse.php?list=category&id=3 and can use at your site. So your custom theme which is based on moodle core theme will not be work in Moodle2.7 .