I got it to work, but not with the above solution.
Editing the template of the plugin doesn’t work because of the style pack software that’s being used. Instead, I had to add custom CSS to the style pack software.
Benefit of this is that it should persist through updates (the template folder of the plugin would be reset every update, requiring a change every time).
Any color can be chosen for the text color of closed threads. I chose #8B0000 from this chart: https://www.w3schools.com/cssref/css_colors.php
The code used is
#bbpress-forums ul.status-closed,
#bbpress-forums ul.status-closed a {
color: #8B0000;
}
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