Custom theme select box placement is confusing on some blocks

To reproduce:

1. Create a block via admin/build/block/add
2. Edit the block that you just created
3. Notice that in the Block Specific Settings fieldset, the "Save block" button is above the "Custom theme" select box

When blocks are exposed via other modules, this doesn't appear to be an issue because there is no "Save block" button in that fieldset.

The fix is to add:

     '#weight' => -1,

to the $form['block_settings']['blocktheme'] array in blocktheme_form_alter().