How do I choose a default theme if the theme gives a white screen?
You can update in database.

Change the active theme in the database

The default theme setting is stored in Drupal's database, and thus can be changed by manipulating the database from the command line or an administration overlay or client like PHPMyAdmin or MySQL Workbench for MySQL.

Always make a backup of your database before changing values in it directly, as it is possible to destroy data and render your site unrecoverable at the database level. You have been warned.

In Drupal 6 and 7, the setting is stored in the system table:

activate a trusted theme

UPDATE system SET status=1 WHERE name = 'garland';


change the default setting

UPDATE variable SET value='s:7:"garland"' WHERE name = 'theme_default';


clear the cache tables

TRUNCATE cache;
TRUNCATE cache_bootstrap;
TRUNCATE cache_block;


Another way

sites/default/settings.php:
$conf['theme_default'] = 'bartik';