Dark theme for dropdowns in the Web IDE

What does this MR do?

Issue #213831 (closed)

Adds dark theme support for the dropdowns (and tooltips) in the web IDE

• Select branch/MR dropdown
• File row dropdown
• File/File template dropdown (for certain files like .gitignore)
• Review dropdown (to compare changes with the MR or last changes)

Screenshots

Area	Before	After
Select branch
Select MR
File row
Review/Edit
File Template 1
File Template 2
Commit message tooltip
File search box (opened by Cmd+P)

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