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FormKit ships robust and accessible markup — but with no assumptions about your desired styles. There is an optional base theme (as seen in these docs) called Genesis that you can use in your projects.

Installing Genesis theme

To install Genesis, first install the @formkit/themes package.

Then in your main.js (wherever you boot Vue up) include the Genesis theme.css (this assumes you are using a build tool like Vite, Webpack, Snowpack, or Nuxt):

Outer attributes

For styling purposes some attributes are automatically added to and removed from the outer section of all FormKit inputs:

  • data-type — The type of input, text, select, checkbox etc.
  • data-multiple — For inputs that accept the multiple attribute, this will be appended when the input has the multiple attribute (like the select input).
  • data-disabled — Present when an input is disabled.
  • data-complete — Present when the input is "complete". Intended to be used for styling the input when a user has completed filling out the input (like a green checkmark). Read about context.state.complete for information what conditions cause this to be true.
  • data-invalid — Present when the input has failing validation rules and the messages for the failing rules are visible.
  • data-errors — Present when the input has explicitly set errors.

You can use the above attributes to easily provide realtime visual feedback for users filling out your forms:

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Custom classes

Most users will want to apply their own styles and classes to FormKit's provided markup. FormKit provides numerous methods to apply classes for your project.

Classes can be modified for all sections using any of the following methods (from highest to lowest specificity):

The classes follow a strict hierarchy. Initially, classes are produced by the rootClasses function. They can then be modified by the classes configuration option, then by the classes prop, and finally by the {section-key}-class prop. At each of these stages classes can be appended, reset, or selectively modified.

Appending classes

To append a class, simply return the string you want to append, or provide an object of classes with boolean values — true properties will be appended:

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Resetting classes

Classes produced by all earlier hierarchy steps can be completely removed by providing a special (not rendered) class $reset in either string format or object format:

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Removing classes

Classes produced by an earlier step in the class hierarchy can be selectively removed by providing an object with the value false for the class you want to remove. This includes removing formkit's default formkit- prefixed classes:

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In addition to the four methods listed above, more generalized overrides are also available, like overriding an input’s schema, using the classes node hook, or utilizing slots:

Section-key class props

The simplest way to modify the classes of an element inside a FormKit input is via the {section-key}-class props. To add a class to a specific section element, like label, you simply add the label-class prop:

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Classes prop

The classes prop is similar to the section-key class prop except it allows setting classes on all sections at the same time:

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Classes configuration

The classes configuration option is similar to the classes prop, except it applies to all inputs the configuration is applied to. FormKit's unique configuration system allows for you to apply classes globally on your project or just inputs within a certain group or form:

Global class configuration

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Class configuration on a group, list, or form

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Using generateClasses from @formkit/themes

FormKit ships with a helper function called generateClasses included in @formkit/themes.

The generateClasses function takes a javascript object keyed by input type with values of a sub-object keyed by ${sectionKey} with values of strings. With this function you can quickly apply class lists to sections within inputs based on a given inputs' type.

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The rootClasses function

rootClasses is a configuration function that is responsible for producing the default classes for each element. This function already has a default value which produces all the default classes (like formkit-outer and formkit-label) that ship with FormKit — so replacing this single function allows you to easily replace all initial classes. This makes it an ideal candidate for writing custom themes when using utility frameworks like Tailwind.

The rootClasses function is passed 2 arguments (respectively):

The function will be called once for each section and it must return an object of classes with boolean values.

While typical usage of rootClasses is at the global config level to apply classes to your entire project - you can also use it with the config prop to override a specific form or input within your project with a class list computed from the logic within your provided function:

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Because rootClasses is a configuration option, you can apply it per input, per group, or globally.

Modifying classes within schema

In addition to modifying classes via config or props on a <FormKit> component, you can use the same techniques within schema:

Section-key class props within schema

Within schema, you can also modify the classes of an element inside an input via the {section-key}Class properties. For example, to add a class to the label section, you can add the labelClass property:

Classes prop within schema

Much like the classes prop on a <FormKit> component, you can modify the class list for any section of an input with the classes property on a schema node:

Config within schema

Since config is passed down to descendant inputs, you can alter classes via config on a parent, such as a form, list, or a group, and this will affect all descendants to any depth:

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Tailwind CSS

Want to use Tailwind with FormKit? We've written a guide walking through the process using both inline class props as well as using the FormKitVariants plugin generateClasses helper function from @formkit/themes` to create a global Tailwind theme:

Guide: Create a Tailwind CSS theme Read now