I know this was a while ago, but in case anyone else stumbles across this topic, here’s how I did it.
In your wordpress install find the folder /wp-content/plugins/bbpress/templates/default/css
Open the file bbpress.css in a text editor
Within the bbpress.css file, search for the text #bbpress-forums .status-closed
When you find this text, below is the entry color: #ccc; This is the colour for your text in closed topics. Change it to #000 for black.