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.
