/**
 * Customizer for extendTheme that merges class strings instead of replacing them.
 * Pass as the third argument to extendTheme to append/override specific Tailwind
 * classes rather than wholesale replacing a theme property.
 *
 * @example
 * extendTheme(theme, customTheme, mergeThemeClasses)
 */
export declare function mergeThemeClasses(source: any, target: any): string | undefined;