Theme callback: Uses the theme specified in the parameter.
$theme: The theme whose blocks are being configured. If not set, the default theme is assumed.
The theme that should be used for the block configuration page, or NULL to indicate that the default theme should be used.
function _block_custom_theme($theme = NULL) {
// We return exactly what was passed in, to guarantee that the page will
// always be displayed using the theme whose blocks are being configured.
return $theme;
}