How can I insert a variable from a preprocess function in template.php to Block Body in admin
You can't do that.

The block content doesn't ahve access to variables you add in the block preprocess function.
The block preprocess function code runs after the block content has been generated.

Since you have a PHP block just add that code from your preprocess function directly to it.

So put this at the top of your block content:

$theme_url = base_path() . drupal_get_path('theme', variable_get('theme_default', NULL));