NOTE: Originally reported at https://www.mediawiki.org/wiki/Extension:Challenge#Issues.2FTo-do
Haven't tested yet, but apparently ChallengeHistory doesn't have this icon yet. I'm assuming it means a check mark, so I recommend using https://www.mediawiki.org/wiki/Design/WikiFont#/media/File:Tickmark_icon.svg and upload the svg into the Challenge's source code since that's part of WikiFont.
The latest version of that icon is available on wikimedia/WikiFont at github, https://github.com/wikimedia/WikiFont/blob/master/svgs/uniE022%20-%20check.svg. If it needs to be green, then use the fill attribute and make it equivalent to GREEN50 (#00af89, rgb(0, 157, 137)), which is the official green of the WikimediaUI color palette (M82).