docs: document required CSS custom properties (shadcn/ui design tokens)

[sleitor]

Fixes #449

Problem

Streamdown components rely on shadcn/ui CSS custom properties (--background, --foreground, --card, --border, --radius, etc.) for styling. Without these variables, components render with missing backgrounds, borders, and spacing.

The README installation guide does not mention this dependency, leaving users with broken styling and no indication of what's missing.

Fix

Added a "CSS Custom Properties (Design Tokens)" section to the installation guide in README.md that:

• Explains the shadcn/ui CSS variable dependency
• Provides a minimal copy-paste CSS snippet for users not using shadcn/ui
• Links to the shadcn/ui theme generator for custom palette creation

[vercel]

Someone is attempting to deploy a commit to the Vercel Team on Vercel.

A member of the Team first needs to authorize it.

[haydenbleasel]

good call, this has been assumed knowledge so far. appreciate it!