Restores the `$is_block_theme` check that was inadvertently removed in [61473]. This ensures that for classic themes, the Customizer's Additional CSS continues to be printed separately via `wp_custom_css_cb()` at priority 101 in `wp_head`, preserving its position at the end of the `<head>` for highest CSS specificity. Follow-up to [61473]. Props westonruter. Fixes #64408. Built from https://develop.svn.wordpress.org/trunk@61479 git-svn-id: http://core.svn.wordpress.org/trunk@60791 1a063a9b-81f0-0310-95a4-ce76da25c4cd