Удалите thead внутри tfoot, tbody и используйте только tr и th и добавьте розовый цвет фона
Примечание : элементы thead и tfoot можно использовать в таблице один раз.
tbody > tr > th, tfoot > tr > th {background-color: skyblue !important; } /*0005 - to "Final students marks" and "Avarage marks in groups"*/
tbody > tr > td:first-child, tfoot > tr > td:first-child {
background-color: yellow
}
пример кода для справки
body {
background-color: black;
text-align: center;
color: black;
}
table > thead {background-color: pink;} /*0002 - to main thead*/
tbody > tr:nth-child(even) {background: #CCC;} /*0011*/
tbody > tr:nth-child(odd) {background: #FFF;} /* why it styling tfoot?? */
tfoot tr:nth-child(even) {background: sandybrown !important; /*why not work???*/ }
tfoot tr:nth-child(odd) {background: sandybrown !important;}
tbody > tr > th, tfoot > tr > th {background-color: skyblue !important; } /*0005 - to "Final students marks" and "Avarage marks in groups"*/
tbody > tr > td:first-child, tfoot > tr > td:first-child {
background-color: yellow
}
<head>
<title>Page Title</title>
</head>
<body>
<table>
<thead>
<tr><th>Number</th><th>Name</th><th>Surname</th><th>Group</th><th>Mark</th></tr>
</thead>
<tbody>
<tr><th colspan="5">Final students marks</th></tr>
<tr><td>1</td><td>Adam</td><td>Abacki</td><td>A</td><td>4</td></tr>
<tr><td>2</td><td>Ewa</td><td>Babacka</td><td>B</td><td>5</td></tr>
<tr><td>3</td><td>Edward</td><td>Cabacki</td><td>A</td><td>3</td></tr>
<tr><td>4</td><td>Tomasz</td><td>Dadacki</td><td>A</td><td>4</td></tr>
<tr><td>5</td><td>Anna</td><td>Kowalska</td><td>B</td><td>3</td></tr>
<tr><td>6</td><td>Marek</td><td>Zawadzki</td><td>A</td><td>5</td></tr>
</tbody>
<tfoot>
<tr><th colspan="5">Average marks in groups</th></tr>
<tr><td>1</td><td colspan="2"></td><td>A</td><td>4</td></tr>
<tr><td>2</td><td colspan="2"></td><td>B</td><td>4</td></tr>
</tfoot>
</table>
</body>
codepn - https://codepen.io/nagasai/pen/jQwWPm