How can I add dynamic CSS styles?

data- attributes

If the value that controls dynamic styles is a boolean or a known small union of values, use data- attributes.

First, define data- attributes on the component. In the example below:

data-disabled represents the boolean disabled attribute. disabled || undefined is required to not add data-disabled="false" attribute when disabled is false and allow styling with the &[data-disabled] selector.
data-orientation represents the orientation prop which can be either horizontal or vertical. In styles, you can reference it with the &[data-orientation="horizontal"] selector.

import { Box } from '@mantine/core';
import classes from './Demo.module.css';

interface DemoProps {
  disabled: boolean;
  orientation: 'horizontal' | 'vertical';
}

function Demo({ disabled, orientation }: DemoProps) {
  return (
    <Box
      data-disabled={disabled || undefined}
      data-orientation={orientation}
      className={classes.root}
    >
      My demo
    </Box>
  );
}

Then add styles in the .module.css file:

.root {
  background: orange;
  display: flex;

  &[data-disabled] {
    background: silver;
  }

  &[data-orientation='horizontal'] {
    flex-direction: row;
  }

  &[data-orientation='vertical'] {
    flex-direction: column;
  }
}

Inline styles

If the value that controls dynamic styles is not represented by a known union of values (for example, the value can be any valid CSS color), then you can use inline styles or style props:

import { Box } from '@mantine/core';

interface DemoProps {
  fontFamily: string;
  color: string;
}

function Demo({ fontFamily, color }: DemoProps) {
  return (
    <Box style={{ backgroundColor: color }} ff={fontFamily}>
      My demo
    </Box>
  );
}

If you need to customize a deeply nested element, use the styles prop instead:

import { Button } from '@mantine/core';

interface DemoProps {
  color: string;
}

function Demo({ color }: DemoProps) {
  return (
    <Button styles={{ label: { backgroundColor: color } }}>
      My demo
    </Button>
  );
}

Note that it is not possible to use pseudo-classes (for example, :hover, :first-of-type) and media queries inside the styles prop. For this purpose, use CSS variables with the classNames prop.

CSS variables

If none of the methods above work for you (for example, you want to customize :hover styles based on component prop), use CSS variables with the classNames prop.

First, define CSS variables in the style or styles prop:

import { Box } from '@mantine/core';
import classes from './Demo.module.css';

interface DemoProps {
  color: string;
}

function Demo({ color }: DemoProps) {
  return (
    <Box style={{ '--demo-hover': color }} className={classes.root}>
      My demo
    </Box>
  );
}

Then reference them in the .module.css file:

.root {
  background: orange;

  &:hover {
    background: var(--demo-hover);
  }
}