import type { ProjectBackground } from '../types/timeline.js';
/**
 * CSS value for a project background, usable directly as the `background`
 * shorthand. `null` resolves to `'transparent'` (no background); the default
 * project background is solid black, so the historical look is preserved unless
 * the user explicitly chooses transparent.
 */
export declare function backgroundCss(bg: ProjectBackground | null): string;