Can I bold the last three lines of "lightning-datatable" in a LWC?

Question

//html

<div class="last-row-bold">
    <lightning-datatable
            class="slds-table_striped"
            key-field="Id"
            data={dataForDisplay}
            columns={columns}
            hide-checkbox-column
            onheaderaction={handleHeaderAction}>
    </lightning-datatable>
</div>

I saw this question and tried to do the same as the answer, but it doesn't work for me...

//css

.THIS.last-row-bold table tr:last-child * {
    font-weight: 800;
}

If I use this CSS, I get an error:

.THIS.last-row-bold table tr:nth-last-child(-n+3) * {
    font-weight: 800;
}

Error:

Invalid Descriptor Format: :n[MODULE]

This is what my table looks like and I need to highlight the last three rows (which I highlighted in red):