Nevermind, got it fixed. added #bbpress-forums id inside the class’ not() selector.
.main-content .entry-content:not(#BP, #bbpress-forums) ul li {
font-size:12px;
line-height:19px;
margin: 3px 0;
background: url(images/light/arrow-right-grey.png) no-repeat 0 10px;
border-bottom: 1px solid
#F1F1F1;
padding: 5px 0 5px 15px;
width: auto;
display: list-item;
}