Custom styles in Tailwind CSS: `@apply`, `theme` or custom plugins

There are three ways to add custom styles to a Tailwind CSS project. As there have been some recent tweets around one of them - the @apply directive - I'd like to look at and give examples for each.

What is @apply?

@apply is a PostCSS directive, provided by Tailwind, to allow re-using it's classes - either when extracting components or overriding third-party styles.

The CSS file is the same as if you were writing traditional CSS, but rather than adding declarations to a ruleset, you use the @apply directive and specify the Tailwind CSS class names that you want to apply.

For example:

fieldset {
@apply bg-primary-dark;
}

This is a simple example but it's easy to see how this could be used in ways that weren't intended and how edge-cases can be found.

Adam said in a another tweet:

I estimate that we spend at least $10,000/month trying to debug extremely edge-case issues people run into by using @apply in weird ways.

Using the theme function

As well as @apply, Tailwind also provides a theme function that you can use in your CSS file. This removes the abstraction of using the class names and adds the ability to retrieve values from the theme section of your tailwind.config.js file.

fieldset {
backgroundColor: theme('colors.primary.dark');
}

This seems to be the preferred approach over using @apply.

Creating a custom plugin

The theme function is also available if you write a custom Tailwind CSS plugin:

const plugin = require('tailwindcss/plugin')

plugin(({ addBase, theme }) => {
addBase({
fieldset: {
backgroundColor: theme('colors.primary.dark'),
}
})
})

This is an approach that I've used for generic, open-source plugins but for project-specific styling, I've mostly used @apply or the theme function.

That said, I like the modular architecture of having different custom plugins - especially if they're separated into their own files - and being able to easily toggle plugins by simply adding to or removing from the plugins array.

I usually don't write many custom styles in a Tailwind project but I think that I'll focus on using the theme function going forward, either in a stylesheet or a custom plugin.