Is this working? Icons instead of styles?

Doesn’t seem to be working for me either.
Once it does, here is how I used icons:

.bbp-body .hentry.bbppu-read>li:first-child{
padding-left: 40px;
height: 40px;
background: transparent url(https://) no-repeat;
border-left:0px solid transparent;
}

.bbp-body .hentry.bbppu-unread>li:first-child{
padding-left: 40px;
height: 40px;
background: transparent url(https://) no-repeat;
border-left:0px solid transparent;
}

You can add that to the CSS file, but each time it updates you will need to replace it.

I got them to show by adding the !important tag to the CSS:

.bbp-body .hentry.bbppu-read>li:first-child{
padding-left: 40px;
height: 40px;
background: transparent url(https://) no-repeat;
border-left:0px solid transparent !important;
}

.bbp-body .hentry.bbppu-unread>li:first-child{
padding-left: 40px;
height: 40px;
background: transparent url(https://) no-repeat;
border-left:0px solid transparent !important;
}

As angelladesigns, yes you can use icons.
Once you have those css classes enabled, everything is possible !