This is still an issue in CodeMirror 6.

r980541 used a strange selector for the matching brackets, and I am not sure whether it was intentional.

.cm-matchingBracket,
.cm-focused .cm-matchingBracket {
	background-color: #eee;
	box-shadow: inset 0 0 1px 1px #999;
	font-weight: bold;
}

Change #1226296 had a related patch set uploaded (by 沈澄心; author: Prashant_32194):

[mediawiki/extensions/CodeMirror@master] Fix dark-mode bracket visibility in CodeMirror selections

https://gerrit.wikimedia.org/r/1226296

@Prashant32194 Thanks for your patch! Just a reminder: The commit message needs to be Bug: T402293 instead of Bug Id: T402293 so that gerritbot can function properly.