In Boostrap 4 how can I change the the translucency of toasts to fully opaque?

I am using Boostrap 4.6 (and I cannot upgrade to BS 5 yet). How can I change the translucency of toasts to fully opaque?

My toast looks like this:

    <div id="help" style="position: fixed; top: 0.5vw; bottom: 0.5vw; right: 10vw;">
        <div class="toast" role="alert" aria-live="assertive" aria-atomic="true">
            <div class="toast-header">
                <strong class="mr-auto">Help for <%=systemName%></strong>
                <button type="button" class="ml-2 mb-1 close" data-dismiss="toast" aria-label="Close">
                    <span aria-hidden="true">&times;</span>
                </button>
            </div>
            <div class="toast-body">
                hallo
                <!-- <zero-md src="help/hallswell_all_towers.md"></zero-md> -->
            </div>
        </div>
    </div>

I tried to add in our CSS style sheet (which is loaded after the Bootstrap CSS) the following:

.toast {
  opacity: 1;
}

but this does not have the desired effect.

So, how can I change the translucency of toasts?