How can I let html table borders merge into each other in css

In a current project I have a three-columned table.

<table>
    <tr>
        <th>Datum</th>
        <th>Entschuldigt?</th>
        <th>Vermerkt von</th>
    </tr>
</table>

My current stylesheet for the table looks as following:

th {
    border-bottom: 1px solid black;
}

td + td {
    border-left: 1px solid black;
    border-right: 1px solid black;
}

Now the table looks like this

Now I would like to have those little spaces removed so the column separators and the header separator are solid and not that kind of dashed. Every help is very much appreciated