/**
 * Converts an hexadecimal colour into CSS RGB/A colour.
 * @param {string} hex - An hexadecimal colour in the format:
 *
 * - `#000`
 * - `#102030`
 * - `#ffff`
 * - `#102030ff`
 *
 * @returns {string} either a CSS RGB or CSS RGBA string.
 */
export declare function convertHextoCSSRGB(hex: string): string | null;