[bug]: shimmer button text is also black in dark mode, making it invisible

Hello, I found that when using `magicui mcp`, the `ShimmerButton` has an issue in dark mode where the text color is similar to the background, making it invisible.  
Usage example:  

```tsx
<ShimmerButton>Shimmer Button</ShimmerButton>
```

<img width="183" height="70" alt="Image" src="https://github.com/user-attachments/assets/c33e37c3-cf41-4123-9dad-e60c108111e5" />