можно привязаться к атрибуту rowspan
вместо
#text_scheduleBox tr:nth-child(2n+3){
background-color: #FFAFB1;
}
#text_scheduleBox tr:nth-child(2n) td:nth-child(n+2){
background-color: #ADCFFF;
}
использовать
#text_scheduleBox td:not([rowspan]) {
background-color: #FFAFB1;
}
#text_scheduleBox td[rowspan] ~ td{
background-color: #ADCFFF;
}
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style type="text/css">
#text_scheduleBox table, #text_scheduleBox th {
text-align: center;
vertical-align: middle;
margin: 1%;
padding: 1.5%;
border: 1px solid black;
}
#text_scheduleBox td {
width: auto;
margin: 1%;
border: 1px solid black;
}
#text_scheduleBox td:not([rowspan]) {
background-color: #FFAFB1;
}
#text_scheduleBox td[rowspan] ~ td{
background-color: #ADCFFF;
}
</style>
</head>
<body>
<div id="text_scheduleBox">
<fieldset>
<legend> OUR SCHEDULE </legend>
<table>
<tr>
<th>Time</th>
<th>Monday</th>
<th>Tuesday</th>
<th>Wednesday</th>
<th>Thursday</th>
<th>Friday</th>
<th>Saturday</th>
<th>Sunday</th>
</tr>
<tr>
<td rowspan="2">9-00...11-00</td>
<td>Abraham</td>
<td>Dmitry</td>
<td>Abraham</td>
<td>Dmitry</td>
<td>Eugene</td>
<td>Oleg</td>
<td>Dmitry</td>
</tr>
<tr>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>Abraham</td>
<td>Evgeniya</td>
</tr>
<tr>
<td rowspan="2">11-00...13-00</td>
<td>Abraham</td>
<td>Oleg</td>
<td>Abraham</td>
<td>Oleg</td>
<td>Eugene</td>
<td>Oleg</td>
<td>Dmitry</td>
</tr>
<tr>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>Abraham</td>
<td>Evgeniya</td>
</tr>
<tr>
<td rowspan="2">13-00...15-00</td>
<td>Abraham</td>
<td>Dmitry</td>
<td>Abraham</td>
<td>Dmitry</td>
<td>Eugene</td>
<td>Oleg</td>
<td>Dmitry</td>
</tr>
<tr>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>Abraham</td>
<td>Evgeniya</td>
</tr>
<tr>
<td rowspan="2">15-00...17-00</td>
<td>Abraham</td>
<td>Oleg</td>
<td>Eugene</td>
<td>Oleg</td>
<td>Eugene</td>
<td>Dmitry</td>
<td>Dmitry</td>
</tr>
<tr>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
</tr>
<tr>
<td rowspan="2">17-00...19-00</td>
<td>Eugene</td>
<td>Oleg</td>
<td>Eugene</td>
<td>Oleg</td>
<td>Eugene</td>
<td>Dmitry</td>
<td>Dmitry</td>
</tr>
<tr>
<td>-</td>
<td>Abraham</td>
<td>Evgeniya</td>
<td>Abraham</td>
<td>Evgeniya</td>
<td>-</td>
<td>Evgeniya</td>
</tr>
<tr>
<td rowspan="2">19-00...21-00</td>
<td>Eugene</td>
<td>Dmitry</td>
<td>Eugene</td>
<td>Dmitry</td>
<td>Eugene</td>
<td>Dmitry</td>
<td>Dmitry</td>
</tr>
<tr>
<td>-</td>
<td>Abraham</td>
<td>Evgeniya</td>
<td>Abraham</td>
<td>Evgeniya</td>
<td>-</td>
<td>Evgeniya</td>
</tr>
<tr>
<td rowspan="2">21-00...23-00</td>
<td>Eugene</td>
<td>Dmitry</td>
<td>Eugene</td>
<td>Dmitry</td>
<td>Eugene</td>
<td>Dmitry</td>
<td>Dmitry</td>
</tr>
<tr>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
</tr>
</table>
</fieldset>
</div>
</body>
</html>