Source

File: wp-includes/class-wp-block-styles-registry.php

public function register( $block_name, $style_properties ) {

		if ( ! isset( $block_name ) || ! is_string( $block_name ) ) {
			$message = __( 'Block name must be a string.' );
			_doing_it_wrong( __METHOD__, $message, '5.3.0' );
			return false;
		}

		if ( ! isset( $style_properties['name'] ) || ! is_string( $style_properties['name'] ) ) {
			$message = __( 'Block style name must be a string.' );
			_doing_it_wrong( __METHOD__, $message, '5.3.0' );
			return false;
		}

		$block_style_name = $style_properties['name'];

		if ( ! isset( $this->registered_block_styles[ $block_name ] ) ) {
			$this->registered_block_styles[ $block_name ] = array();
		}
		$this->registered_block_styles[ $block_name ][ $block_style_name ] = $style_properties;

		return true;
	}