@mui/material/styles/createMixins.d.ts

import * as CSS from 'csstype';
import { Breakpoints } from '@mui/system';

export type NormalCssProperties = CSS.Properties<number | string>;
export type Fontface = CSS.AtRule.FontFace & { fallbacks?: CSS.AtRule.FontFace[] };

/**
 * Allows the user to augment the properties available
 */
export interface BaseCSSProperties extends NormalCssProperties {
  '@font-face'?: Fontface | Fontface[];
}

export interface CSSProperties extends BaseCSSProperties {
  // Allow pseudo selectors and media queries
  // `unknown` is used since TS does not allow assigning an interface without
  // an index signature to one with an index signature. This is to allow type safe
  // module augmentation.
  // Technically we want any key not typed in `BaseCSSProperties` to be of type
  // `CSSProperties` but this doesn't work. The index signature needs to cover
  // BaseCSSProperties as well. Usually you would use `BaseCSSProperties[keyof BaseCSSProperties]`
  // but this would not allow assigning React.CSSProperties to CSSProperties
  [k: string]: unknown | CSSProperties;
}

export interface Mixins {
  toolbar: CSSProperties;
  // ... use interface declaration merging to add custom mixins
}

export interface MixinsOptions extends Partial<Mixins> {
  // ... use interface declaration merging to add custom mixin options
}

export default function createMixins(breakpoints: Breakpoints, mixins: MixinsOptions): Mixins;