:: how to reset wordpress theme to default
# using mysql cli

$ mysql -u user -p
mysql> use dbwordpress;
mysql> UPDATE wp_options SET option_value = 'default' WHERE option_name = 'template';
mysql> UPDATE wp_options SET option_value = 'default' WHERE option_name = 'stylesheet';

## share hosting / cpanel / phpmyadmin
0. login on cpanel / phpmyadmin
1. select db for wordpress
2. select SQL tabs and run this command:

UPDATE wp_options SET option_value = 'default' WHERE option_name = 'template';
UPDATE wp_options SET option_value = 'default' WHERE option_name = 'stylesheet';

3. click GO
4. open your site, if you still get blank screen, you can login on admin page :

//yourdomain.tld/wp-login.php

5. and select your new themes for wordpress.

:: links
+ google
+ toxpenguin