I see. As it so happens, you are already using the property required for this. Right now, you have:
.voicebank-table th,
.voicebank-table td {
padding: 0.2em;
}
Here, you are setting the value for your headers and data cells at the same time to the same thing. If you want to increasing the spacing in your header cells, you will need to separate this rule like this:
.voicebank-table th {
padding: 0.2em;
}
.voicebank-table td {
padding: 0.2em;
}
Now you can change the value for your header cells without changing the value for your data cells. The larger the number, the more spacing there will be between the border of the cells and their contents. Currently, you are setting the spacing to be the same for the top, right, bottom, and left borders. If you want to have the spacing for the top and bottom borders be different from that for the right and left borders, you need to specify two values. For example:
.voicebank-table th {
padding: 0.4em 0.2em;
}
.voicebank-table td {
padding: 0.2em;
}