Source

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

public function _css_href( $src, $ver, $handle ) {
		if ( ! is_bool( $src ) && ! preg_match( '|^(https?:)?//|', $src ) && ! ( $this->content_url && 0 === strpos( $src, $this->content_url ) ) ) {
			$src = $this->base_url . $src;
		}

		if ( ! empty( $ver ) ) {
			$src = add_query_arg( 'ver', $ver, $src );
		}

		/**
		 * Filters an enqueued style's fully-qualified URL.
		 *
		 * @since 2.6.0
		 *
		 * @param string $src    The source URL of the enqueued style.
		 * @param string $handle The style's registered handle.
		 */
		$src = apply_filters( 'style_loader_src', $src, $handle );
		return esc_url( $src );
	}