If you are building a custom WordPress theme for a client’s website or your personal project you can custom-tailor every detail because you know who will be using your theme, what type of content will be managed and any other specific requirements. Your theme may not need to incorporate some WordPress functionality, like widgets, custom header images or background, post thumbnails and so on. But if you are building a theme that will be submitted to WordPress.org themes directory or sold through theme marketplace sites your theme needs to have a certain minimum level of capabilities, in order to ensure proper functioning on a wide variety of sites for a wide range of users.
Luckily there’s a great tool to test if your theme supports the latest WordPress theme standards and practices. It is the Theme Check plugin.
Theme Check looks for things like deprecated functions, required template tags or WordPress-Generated CSS classes and other things that are required or recommended according to the latest WordPress Theme Review guidelines. The tests are run through a simple admin menu and all results are displayed at once:
Download plugin from the WordPress.org plugin directory.