Fix delete board dropdown text color

What does this MR do?

• Fix "Delete board" dropdown text color (and hover)

Before	After

Are there points in the code the reviewer needs to double check?

• Should the text be red? https://gitlab.com/gitlab-org/gitlab-ee/issues/3126#note_37121747
  • Yes

Why was this MR needed?

The hover color was off, too light, and didn't match the other dropdown items.

Does this MR meet the acceptance criteria?

• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #3126 (closed)