Custom Gradient Picker: Add border to improve visibility of light gradients

[dhruvikpatel18]

Closes #50507

What?

Adds a visible border and subtle box shadow to the gradient picker bar to improve visibility when using white or light-colored gradients.

Why?

Currently, when the gradient is white or very light in color, the field in the color palette panel is hard to see, making it difficult for users to interact with the gradient controls. This creates a poor user experience when working with light colors.

How?

• Added a default border using WordPress admin theme color
• Implemented a dynamic border that becomes more prominent for light gradients
• Added subtle box shadow for better definition
• Maintained existing checkerboard pattern for transparency
• Added utility function to detect light gradients
• Added appropriate className conditionals

Testing Instructions

1. Open the block editor
2. Add any block that supports gradients (like a Cover block)
3. Open the gradient picker
4. Create a gradient using white or very light colors
5. Verify that the gradient bar is clearly visible with a border
6. Create another gradient with dark colors
7. Verify that the border is appropriately subtle for dark gradients
8. Test different gradient combinations to ensure the border visibility is appropriate

Screenshots or screencast

Before

Screen.Recording.2025-02-19.at.12.34.24.PM.mov

After

Screen.Recording.2025-02-19.at.12.35.29.PM.mov

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Co-authored-by: dhruvikpatel18 <[email protected]>
Co-authored-by: hanneslsm <[email protected]>

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