Improve CodeMirror contrast & readability with basic WCAG standards (Dark theme)

[cameronhum]

Description

Adjusted dark theme to meet WCAG guidelines for comments numbers selections. Did not touch active line as pushing this contrast any further will break the "subtle" requirement

• Comments meet WCAG AAA pass for normal text 7.07
• selection WCAG AA graphical object and user interface components 3.03:1
• editor number already AA but pushed to WCAG AAA 7.05:1

made all minimal to ensure theme looks the same but with visible readability

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can
reproduce. Please also list any relevant details for your test configuration

Testing Checklist

• Tested in latest Chrome
• Tested in latest Safari
• Tested in latest Firefox

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have requested a review from ... on the Pull Request