Problema com Tema do Wordpress

O site desenvolvido em Wordpress apresentou a tela branca da morte e não estou conseguindo reverter. Já desabilitei os plugins, mas o problema parece ser o tema.

Ao habilitar o debug aparece os seguintes erros:
Notice: Constant WP_MEMORY_LIMIT already defined in /home/ftp.pier8/site_pier/wp-config.php on line 92
Fatal error: Uncaught Error: Call to undefined function mb_strtolower() in /home/ftp.pier8/site_pier/wp-content/themes/revolution/inc/admin/option-tree/ot-functions.php:177 Stack trace: #0 /home/ftp.pier8/site_pier/wp-content/themes/revolution/inc/script-calls.php(26): thb_google_webfont() #1 /home/ftp.pier8/site_pier/wp-includes/class-wp-hook.php(286): thb_main_styles('') #2 /home/ftp.pier8/site_pier/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(NULL, Array) #3 /home/ftp.pier8/site_pier/wp-includes/plugin.php(453): WP_Hook->do_action(Array) #4 /home/ftp.pier8/site_pier/wp-includes/script-loader.php(1447): do_action('wp_enqueue_scri...') #5 /home/ftp.pier8/site_pier/wp-includes/class-wp-hook.php(286): wp_enqueue_scripts('') #6 /home/ftp.pier8/site_pier/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(NULL, Array) #7 /home/ftp.pier8/site_pier/wp-includes/plugin.php(453): WP_Hook->do_action(Array) #8 /home/ftp.pier8/site_pier/wp-includes/general-template.php(2614): do_action('wp_head') #9 /home/ftp.pi in /home/ftp.pier8/site_pier/wp-content/themes/revolution/inc/admin/option-tree/ot-functions.php on line 177