feat(DesignTokens): update overlap background color to color.neutral.900

What does this MR do?

Discussion !4638 (comment 2130006666) and !4623 (comment 2130043749).

Issue: when color.neutral.800 is used for background.color.overlap (used in GlModal and GlPopover) it decreases the contrast between foreground content and surface (background) color. Form field etc. blend into the background.

Proposed solution: change background.color.overlap to color.neutral.900 which mimmicks the previous inverted behaviour of the $white SCSS variable in dark mode. It's safer to assume contents will display as intended on this surface color. Once we have confidence that other tokenisation allows us to change the surface color to align with dark mode vision we can update --gl-background-color-overlap locally in consumers like GitLab to validate and update in GitLab UI to apply universally.

Screenshots or screen recordings

Before	After

Integration merge requests

• GitLab: mr_url
• CustomersDot: mr_url
• Status Page: mr_url

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

Toggle the acceptance checklist

Conformity

• Code review guidelines.
• GitLab UI's contributing guidelines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened (see integration merge requests above).
• Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.