Change appearance of selective column/row separator lines in QTableWidget

I want to separate specific columns/rows in a QTableWidget using a bold colored line.

I could easily achieve a separation by drawing lines on both sides of the the cells adjacent to the column separation (by implementing a custom QItemDelegate):

However, I do not want to draw inside the cells, but rather in the area between them.

Furthermore, an extension of the line into the header would be the ideal appearance for my application:

Is there a way to achieve this look?