docs: #548 - Add details about adding styles in the react project root

[ZtheLeader]

What

Adds missing ModernColors setup step to the "Get Started" documentation for https://wave.free-now.com/?path=/docs/get-started--docs page.

Why

If someone follows the setup instructions on the website without this step, the Wave components appear unstyled, which can create a confusing experience for new users.

How

Included an example usage of ModernColors in the setup guide to ensure proper CSS variables are loaded. Hence it closes #548