Change line style of table borders in OpenOffice.org writer

Hello,

I work with OpenOffice.org 2.0.2 Writer (-ubuntu 12-2) on Kubuntu 6.06 and would like to create a table in which some lines are thicker than others.
When I try to change the line style of table borders via the "table" window the line style of all, not only some, lines of the cell are changed. Marking the cells and using "Paragraph..." or "Edit Paragraph Style..." after a right mouse click results in lines within the cells, not bordering them.

Could anyone please help me with this problem?

Thanks a lot,

Mark