Useful plugins for WordPress theme developers
As a WordPress theme developer, I have installed a number of plugins in my WordPress test environment, which facilitate the development of a theme with useful tools. Today I would like to briefly introduce these WordPress development plugins.
Usually, I always present a single plugin and test and describe it in detail. This allows a more in-depth examination of the features of a plugin and doesn't just scratch the surface. Otherwise you could just read the plugin description on WordPress.org.
For this reason, I find lists of plugins such as “The 10 best membership plugins” or “5 popular plugins for forms” usually not very meaningful, since such posts often contain little information and do not provide any information about what the plugins can actually do.
Nevertheless, I'll make an exception for this post and briefly introduce many plugins.
Most plugins are very small and don't justify their own contribution. In addition, all plugins are intended for WordPress developers, but perform different tasks. It is therefore not a comparison of plugins from the same category, but a list of tools.
WordPress Beta Tester
With this plugin you can upgrade your local WordPress installation to the latest beta version. Manual installation of the beta with each new update is no longer necessary.
As a WordPress developer, you should test your themes and plugins extensively with upcoming WordPress core versions before they appear. This way, bugs and incompatibility with new core features can be excluded.

In addition to your own code, you can also test new core features and report possible bugs. Since the plugin installs an unfinished beta of WordPress Core, the plugin should never be used productively on a live website.
Theme check
With the Theme Check Plugin you can check your WordPress theme for compliance with the WordPress.org Theme Review Guidelines .
This ensures that your theme meets the latest best practices and WordPress theme standards.
The check is especially important if you want to publish your theme on WordPress.org. Every theme uploaded there must pass the automated theme check, which is based on this plugin. In the event of errors, the theme is automatically rejected.
Theme sniffer
The Theme Sniffer Plugin is currently being developed by the WordPress.org Theme Review Team and is the successor to the Theme Check Plugin. The development takes place on Github . The plugin can also be downloaded there.

The plugin is based on the WordPress Coding Standards Rules for PHP_CodeSniffer and thus allows a much better analysis of the source code for compliance with the standards. New rules can thus be implemented faster and easier than before.
Show current template
A very small and useful plugin is Show Current Template :
The plugin shows the activated theme, the name of the current template and all template files that are integrated by it.
Regenerate thumbnails
If you change the image sizes in your theme or define new image sizes, you can use Regenerate Thumbnails to regenerate all existing images for the new image sizes.
You can find detailed instructions for the plugin in my article Regenerating WordPress Featured Images After Changing the Theme .
RTL tester
RTL stands for Right-To-Left and means the direction of writing. For example, Arabic and Hebrew are two languages in which, in contrast to German, people write and read from right to left.
You can use the RTL Tester plug-in to test the functionality of your WordPress theme with RTL Languages.
The plugin adds a button in the WordPress toolbar that changes the text direction to RTL. The whole layout is mirrored in this way.
User switching
The User Switching Plugin enables you to quickly switch between WordPress accounts and display the website as a logged-out guest with just one click.
I already presented the plugin a year ago in the article Change WordPress User Account with One Click .
I mainly use the plugin when developing features for my theme shop. Many pages differ there for guests, logged-in users or existing customers and the plug-in is a useful time saver for testing.
Query Monitor
Query Monitor is a very sophisticated debugging plugin with a lot of functions.
The plugin actually deserves its own contribution instead of a brief mention, but for the sake of completeness I would like to list it here anyway.
The plugin shows a range of information directly in the toolbar or in the frontend: database queries, HTTP requests, fired hooks and filters, template files used, integrated scripts and stylesheets, AJAX and REST API calls. In short, pretty much anything that can be analyzed and used for debugging.
I installed Plugin Theme Check, but it only appears in the plugin listing, nowhere else.
So I can't do the check.
Does that have to do with the fact that the compatibility only exists up to version 4.5.12?
Hi Markus,
The plugin also works with the current WordPress version.
I am just seeing that the plugin description on WordPress.org actually has no description of how to use the plugin.
The menu item for the theme check can be found under Design -> Theme Check in the WordPress backend, not under the settings as is often the case with other plugins.
See also this screenshot: https://themekiller.me/wp-content/uploads/2017/12/theme-check-plugin.png
Many Greetings,
Brian