Fix pipeline page in dark mode

What does this MR do?

Use CSS variables so dark mode colors work on page_bundles/ CSS for some pipeline-related pages. Fixes the most highly visible issues.

Specific fixes:

1. Main pipeline job area background color
2. Sidebar/topbar background on job log pages
3. Sidebar link color slightly lighter
4. Fix hover background for CI status icons (the rectangular pills with CI circles in them)

Doesn't fix some of the border colors as I didn't figure out conversions yet, and think some of those styles button/section styles can just be deleted instead.

Screenshots (strongly suggested)

before	after

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