Fixes discussion timeline icon colors in dark mode

What does this MR do and why?

Fixes the discussion timeline icons colors when in dark mode. Previously they would have no background color and the line between them would not be visible.

Note the color has changed slightly when not in dark mode.

Screenshots or screen recordings

How to set up and validate locally

• Enable dark mode in user preferences
• Visit a merge request

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.

Closes #370576 (closed)