<div class="structitemcontainer-group structitemcontainer-group--sticky">
    <xf:foreach loop="$stickythreads" value="$thread">
    <xf:macro template="thread_list_macros" name="item" arg-thread="{$thread}" arg-forum="{$forum}" />
    </xf:foreach>
</div>

                            <xf:if is="{$stickythreads}">
            <hr class="stickyseparator"/>
        </xf:if>

.stickyseparator {
    height: 30px;
    background-color: #4e9ad1;
    border: 0px;
    margin: 0px;
}

https://xenforo.com/community/resources/separate-sticky-threads-from-normal-ones.5706/