Fix merge request thread’s icon buttons color

What does this MR do?

Currently the buttons look disabled because of the very light gray icons. This aligns the icon’s color with the Pajamas button design specs (https://design.gitlab.com/components/button/).

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• [-] Tested in all supported browsers