Fix CSS for MR widget for dark mode

What does this MR do?

Fixes CSS for MR widget for dark mode since connecting line between stages were not supported in Darkmode.

Effectively color of connecting lines between stages will be changed from #dbdbdb to #404040.

Screenshots (strongly suggested)

before	after

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• I have followed the style guides.

Closes #332207 (closed)