To enable the custom borders feature, set the customBorders option. It could be set as true or initialized as an array with predefined setup.
To initialize Handson table with predefined custom borders, provide cells coordinates and border styles in form of an array:
with row/col pairs: {row: 2, col: 2, left: { /*...*/ }} or with range details: {range: {from: {row: 1, col: 1}, to:{row: 3, col: 4}}, left: { /*...*/ }}