root = Card([header, amountSection, dueDateSection, payButton])
header = CardHeader("Invoice #INV-2024-001", "Freelance Design Services — Acme Corp")
amountSection = Card([amountLabel, amountValue], "sunk", "row", "l", "center", "between")
amountLabel = TextContent("Amount Due", "small-heavy")
amountValue = TextContent("$2,450.00 USD", "large-heavy")
dueDateSection = Card([dueLabel, dueValue], "clear", "row", "l", "center", "between")
dueLabel = TextContent("Due Date", "small-heavy")
dueValue = TextContent("March 15, 2025", "large")
payButton = Buttons([payNowBtn])
payNowBtn = Button("Pay Now", null, "primary", "normal", "large")