Bug Report

• [ x ] Yes, I reviewed the contribution guidelines.
• [ x ] Yes, more specifically, I reviewed the guidelines on how to write clear bug reports.

Describe the current, buggy behavior

When I use wp theme is-active theme_name the exit status is 1 (inactive) when theme_name is a parent theme.

Describe how other contributors can replicate this bug

activate a child-theme and use wp theme is-active twentyfifteen and echo $? to check, whether the parent theme is active

Describe what you would expect as the correct outcome

I would expect, that the parent theme also sets echo $? to 0 (active)

Let us know what environment you are running this on

OS:     Linux 4.9.0-8-amd64 #1 SMP Debian 4.9.144-3.1 (2019-02-19) x86_64
Shell:  /bin/bash
PHP binary:     /usr/bin/php7.3
PHP version:    7.3.3-1+0~20190307202245.32+stretch~1.gbp32ebb2
php.ini used:   /etc/php/7.3/cli/php.ini
WP-CLI root dir:        phar://wp-cli.phar/vendor/wp-cli/wp-cli
WP-CLI vendor dir:      phar://wp-cli.phar/vendor
WP_CLI phar path:       /var/www/clients/client430/web1905/web
WP-CLI packages dir:    /root/.wp-cli/packages/
WP-CLI global config:
WP-CLI project config:
WP-CLI version: 2.4.0