Change references from default theme to WP_FALLBACK_THEME

[MichaelH]

Several places still reference the "default" theme folder.

[MichaelH]

Don't know if these two places should be changed, but will reference them:

In wp-admin/themes.php line there is this reference to default but wonder if changing that to WP_FALLBACK_THEME hinders translation:

<div id="message1" class="updated"><p><?php _e('The active theme is broken.  Reverting to the default theme.'); ?></p></div>

and in wp-includes/theme.php not sure what this is for:

if ( ('WordPress Default' == $name || 'WordPress Classic' == $name) &&
   ('default' == $stylesheet || 'classic' == $stylesheet) ) {

[ryan]

(In [14080]) Use WP_FALLBACK_THEME instead of default. Props MichaelH. see #12846

[nacin]

(In [14485]) Remove WP_FALLBACK_THEME, use WP_DEFAULT_THEME instead. We now only fall back to the default theme if the theme is broken. Incomplete themes that need to inherit templates will inherit them from wp-includes/theme-compat -- this behavior is deprecated. fixes #12846.