To do so proceed to 'Style Editor' screen and add the following code to 'Custom CSS':
table.rnr-c.rnr-b-report.report, table.rnr-c.rnr-b-report.report th, table.rnr-c.rnr-b-report.report td {
border: 1px solid #a0a0a0 !important;
border-collapse: collapse;
}