refactor(sbb-overlay): css refactoring

[DavideMininni-Fincons]

Subtask of #3969

[gemini-code-assist]

Summary of Changes

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This pull request focuses on a significant CSS refactoring for the sbb-overlay component. The primary goal is to improve the maintainability and reusability of overlay styles by extracting global CSS variables and a base mixin into a dedicated global stylesheet. The component's main stylesheet has been updated to consume these new global variables, particularly for controlling display states, resulting in a more modular and consistent styling architecture.

Highlights

• New Global Overlay Stylesheet: A new file, src/elements/overlay/overlay.global.scss, was introduced to centralize global CSS variables and a base mixin specifically for the overlay component, promoting better organization and reusability.
• Overlay Component Refactoring: The src/elements/overlay/overlay.scss component was refactored to utilize the newly defined global CSS variables for managing display properties and other styling, replacing direct CSS declarations with variable usage for enhanced flexibility and consistency.
• Core Stylesheet Updates: The src/elements/core/styles/core.scss file was updated to import the new global overlay stylesheet and adjust the usage of overlay-related mixins, including renaming an existing mixin alias.

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Old styles, tangled mess, New variables bring order, Clean code starts to bloom.

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