Sure, a site admin could add a theme to modify the look of a table. For example, something like this could be used to make the header blue with white text:
.cooked table thead th, .d-editor-preview table thead th {
background: dodgerblue;
color: white;
}
And the following would add borders on the left and right of the cells:
.cooked table td, .d-editor-preview table td {
border-right: 1px solid var(--primary-low);
border-left: 1px solid var(--primary-low);
}