[Discover-Next][UI] Query editor darkmode highlight bad contrast

[kavilla]

Description

Query editor automatically highlights the query, making it difficult to see. Also while highlighting it's not properly styling to a darker text.

Steps to recreate

• Start OpenSearch Dashboards with query enhancements enabled and toggled on. Also select dark mode.
• Navigate to Discover
• Select a language that populates the query editor with a non-empty query by default
• Notice the highlighting:
  

Requirements

• Query editor text is not selected by default when switching languages
• Regardless, if a user highlights the text, the text should still be readable in dark mode.