[Lightbox] Add theme.json support for lightbox overlay background color

[sarthaknagoshe2002]

What?

Closes #78506

This PR adds support for a new overlayBackgroundColor property within the lightbox settings in theme.json. It updates the Image block's lightbox rendering to use a CSS custom property for the overlay background instead of a hardcoded inline style.

Why?

Currently, the Image and Gallery block lightbox feature hardcodes the scrim background color as an inline style (style="background-color: #fff"). This is a major pain point for dark themes or portfolio sites, as it forces theme developers to use !important in their custom CSS to override the white background. By moving this to theme.json and utilizing CSS custom properties, we align the lightbox feature with the standard Gutenberg design tools system and make it much easier for theme authors to customize.

How?

• Added overlayBackgroundColor to the lightbox configuration.
• The image block now checks for an overlayBackgroundColor value in the block level settings (settings.blocks.core/image.lightbox) and falls back to the global settings (settings.lightbox).
• Replaced the hardcoded inline background-color on the .scrim div with a CSS custom property: --wp--lightbox--overlay-background-color.
• Updated packages/block-library/src/image/style.scss to consume the new var(--wp--lightbox--overlay-background-color) with a default fallback of rgb(255, 255, 255).

Testing Instructions

1. Activate a block theme.
2. Open the theme's theme.json file and add a global lightbox background color:

   "settings": {
       "lightbox": {
           "overlayBackgroundColor": "rgba(0, 0, 0, 0.9)"
       }
   }

3. Open a post or page in the editor.
4. Insert an Image block, upload an image.
5. Save the post and view it on the front end.
6. Click the image to trigger the lightbox. Verify that the background color is now the dark, semi-transparent color defined in theme.json.
7. Update your theme.json to define a specific color just for the image block (settings.blocks.core/image.lightbox.overlayBackgroundColor = "red"). Verify that the block-level setting successfully overrides the global setting on the front end.

Screenshots or screencast

Before	After

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Co-authored-by: sarthaknagoshe2002 <sarthaknagoshe2002@git.wordpress.org>
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