Checked checkboxes (- [x]) are visually indistinguishable from unchecked ones

[mrafnadeem-apimatic]

In PR comments and descriptions, checked task list items (- [x]) render almost identically to unchecked ones (- [ ]). The only difference is a very slightly darker appearance, making it very hard to
tell which items are done at a glance.

GitHub's web UI renders checked boxes with a clear visual fill/checkmark, making the distinction obvious.

Expected Behavior

Checked items (- [x]) should display a clearly visible checkmark or filled checkbox, consistent with GitHub's web UI.

Actual Behavior

Checked and unchecked items look nearly identical — the checked state is only marginally darker, with no distinct checkmark or fill.

Steps to Reproduce

1. Open a pull request in VS Code using the GitHub Pull Requests extension.
2. View a PR comment or description containing both checked and unchecked task list items:

   - [x] This task is done
   - [ ] This task is not done
   

3. Compare the rendered output with GitHub's web UI.

Notes

This was previously fixed for both checked and unchecked checkboxes in #5310 (~Jan 2024). This may be a regression

• Extension version: 0.136.0 (not prerelease)
• VSCode Version: 1.116.0
• OS: Windows 11 (10.0.26200)
• GitHub Product: GitHub.com