You can combine several line types and styles. The last image shows how to set thick outer borders (the thick black lines), while any diagonal lines inside the cell will not be touched (grey lines).
可以合并多种线条类型和样式。最后一个图像说明了如何设置粗外边框（黑色粗线），而不影响该单元格内的任何对角线（灰色线条）。