tbody border is not flush with table header - css

Question

I am having trouble making tbody border not extending over the width of thead. Is there a way to fix it? Thanks
PS: There are other problems like the empty ".cell" div not sizing properly, however I'm not sure if you can post multi-issue questions. So let's focus on the issue above.
table {
box-sizing: border-box;
border-collapse: collapse;
margin-bottom: 2000px;
}
th {
background-color: white;
position: sticky;
top: 0;
padding: 0;
border: 0;
}
.cell {
box-sizing: border-box;
display: inline-block;
height: 100%;
width: 100%;
padding: 15px;
background-color: red;
}
th:first-of-type > .cell {
border-radius: 5px 0 0 0;
}
th:last-of-type > .cell {
border-radius: 0 5px 0 0;
}
td {
border: 1px solid blue;
background-color: #fff;
padding: 15px;
}
tbody {
border: 2px solid green;
box-sizing: border-box;
}
<table>
<thead>
<tr>
<th><div class="cell"></div></th>
<th><div class="cell">aaaaaa</div></th>
<th><div class="cell">aaaa</div></th>
</tr>
</thead>
<tbody>
<tr>
<td>aaa</td>
<td>aaaaaa</td>
<td>aaaa</td>
</tr>
<tr>
<td>aaa</td>
<td>aaaaaa</td>
<td>aaaa</td>
</tr>
<tr>
<td>aaa</td>
<td>aaaaaa</td>
<td>aaaa</td>
</tr>
</tbody>
</table>

Answer 1

I'm assuming the rounded corners are the reason why you are placing <div> within the <th>. If you want rounded corners then you need to use the following styles:
table { border-collapse:separate; border-spacing:0; border: 0; }
The demo has some additional changes that can be adjusted.
Demo
table {
border-collapse: separate;
border-spacing: 0;
margin-bottom: 2000px;
border: 0;
}
thead {
position: sticky;
top: 0;
}
th {
border: 1px solid red;
background-color: red;
padding:15px;
}
th:first-of-type {
border-top-left-radius: 6px;
}
th:last-of-type {
border-top-right-radius: 6px;
}
td {
border: 1px solid blue;
background-color: #fff;
padding: 15px;
}
<table>
<thead>
<tr>
<th></th>
<th>aaaaaa</th>
<th>aaaa</th>
</tr>
</thead>
<tbody>
<tr>
<td>aaa</td>
<td>aaaaaa</td>
<td>aaaa</td>
</tr>
<tr>
<td>aaa</td>
<td>aaaaaa</td>
<td>aaaa</td>
</tr>
<tr>
<td>aaa</td>
<td>aaaaaa</td>
<td>aaaa</td>
</tr>
</tbody>
</table>