Dark theme for the Web IDE right sidebar

What does this MR do?

Adds support for dark theme for the Web IDE right sidebar (pipelines, live preview and web terminal).

Pipeline status icons currently have a white background, but I've opened gitlab-svgs!560 (merged) to allow its customization.

Issue: #213826 (closed)

Screenshots

Area	Before	After
Pipelines empty state
Pipelines tab
Pipeline logs
Live preview tab
Web Terminal empty state
Web Terminal tab
All changes committed
Sidebar drag handle hover state

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team