Steps to reproduce the issue

There is an alert message (alert alert-info) and success button whose colour is not nice.
Let's make it white.

Screenshot:

The issue occurs due to this rule:

.alert.alert-info a {
  color: var(--state-info-text);
}

Expected result

The solution is to add an extra rule like that:

.alert.alert-info .btn-success a {color:#fff}
And we will get perfect result:

Dark mode is OK.

System information (as much as possible)

Joomla 5.1.1