src/admin/class-papi-admin-meta-box.php
public function admin_head() {
		if ( ! $this->box->frame ) {
			echo sprintf(
				'<style type="text/css">label[for="%s-hide"],#%s>h2,#%s>button{display:none !important}</style>',
				esc_attr( $this->box->id ),
				esc_attr( $this->box->id ),
				esc_attr( $this->box->id )
			);
		}
	}
public function admin_head() {
		if ( ! $this->box->frame ) {
			echo sprintf(
				'<style type="text/css">label[for="%s-hide"],#%s>h2,#%s>button{display:none !important}</style>',
				esc_attr( $this->box->id ),
				esc_attr( $this->box->id ),
				esc_attr( $this->box->id )
			);
		}
	}
Add custom css for hiding boxes with no frame in screen options.

@return string