feat(GlButton): Update design tokens for darkmode

What does this MR do?

Updates design tokens used for buttons to match darkmode design direction:

• Updates action tokens (Darkmode values only)
• Updates button tokens (Darkmode values only)
• Updates split button to use a 2px gap in darkmode
• Updates toggle background color (Darkmode only, action token)

Screenshots or screen recordings

Before	After
Split button
Disabled button

Buttons darkmode design direction

Toggle update

Before	After

Toggle darkmode design direction

Test

• http://localhost:9001/?path=/story/base-button--all-variants-and-categories
• http://localhost:9001/?path=/story/base-button--dropdown-split-button
• http://localhost:9001/?path=/story/base-toggle--default

Integration merge requests

• GitLab: Draft: GitLab UI integration test for buttons-u... (gitlab!169580 - closed)
• 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.

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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.

Related to gitlab#474922, gitlab-org/gitlab-services/design.gitlab.com#1886