XF 2.1 read/unread node icon

[interforo]

I got this

.node--forum.node--read .node-icon i:before{content: url(https://www.mysite/immagini/icon-read.png);}
.node--forum.node--unread .node-icon i:before{content: url(https://www.mysite/immagini/icon-unread.png);}

how do I set margin and widht into the above code?

tks