The hero automatically uses your Squarespace theme colors. Override with:

.wm-scroll-away-hero-section--generated {
  --wm-scroll-away-hero-bg: var(--siteBackgroundColor); 
  --wm-scroll-away-hero-color: var(--headingLargeColor);     
  --wm-scroll-away-hero-title-color: var(--headingExtraLargeColor);
  --wm-scroll-away-hero-text-color: var(--paragraphMediumColor);
}

Or use custom colors:

.wm-scroll-away-hero-section--generated {
  --wm-scroll-away-hero-bg: #1a1a2e;
  --wm-scroll-away-hero-color: #ffffff;
}