Fix markdown preview styling for dark theme

What does this MR do?

Issue: #214239 (closed)

Markdown preview needed a couple of adjustments in the Web IDE dark theme, especially around borders and some styling that looked quite contrasting to the main text color.

Test out on sample project here: https://gitlab.com/-/ide/project/himkp/sample-files/blob/master/-/files/sample.md

This feature is behind a feature flag. To test:

In the rails console (rails c), enter the following command to enable your feature flag

Feature.enable(:webide_dark_theme)

Screenshots

Area	Before	After	Change
h1, h2, blockquote			border-color: $background
code			background-color: $background; color: $text-color
pre			border-color: $background
diff text			Matched dark theme Monaco colors
Table			color: $text-color; border-color: $background, th -> background-color: $border-color
Mermaid charts			Switched to dark theme for mermaid charts

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