Simply go to appearance > style properties > node icons
Upload your node-sprite some where on your server then go to image forum icon and image forum unread icon.
Put the URL of the image in each image box. To get it to work you simply need to put -32px on the background-position or whatever px amount would move it around enough to appear.
So in the little box that says "Freeform CSS/LESS code" make sure you have this in the first one that says image forum icon:
Code:
background-repeat: no-repeat;
background-position: -32px 0;
In the forum unread icon box make sure you give the position "0 0".
Code:
background-repeat: no-repeat;
background-position: 0 0;
Just play around with background-position until you can position the sprite correctly. You might need more than -32px depending on how the sprite looks. For link forum and page forum you can add more background-position if that's also apart of the sprite to achieve the same effect.
Good luck!