Fix unreadable text in dark-themed editor tooltips

What does this MR do and why?

This MR fixes unreadable headings in dark-themed editor tooltips:

These headings are unreadable only in certain combinations:

• light GitLab color theme + dark syntax highlighting theme.
• dark GitLab color theme + light syntax highlighting theme.

The solution feels a bit hacky, but I didn't find a way how to override these specific styles via themes.

Investigation of the issue

1. I looked for possible color definitions in vscode repo, but the closest I got to changing this color was the following, by applying 'editorHoverWidget.foreground': '#ff0000', which only changes the text around the problematic heading:

2. The problem is essentially caused by these styles:

Light GL theme + dark syntax highlighting	Dark GL theme + light syntax highlighting

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to Security & Compliance -> Policies and create a new policy. Example
2. Switch to .yaml mode
3. Hover over the code with warnings

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #371333 (closed)