setBorderXxx() removes default grey cell outline on other sides

[Zihao Yu]

If I call setBorderXxx() explicitly, the grey outline on other sides will be
removed. For instance, I call

style.setBorderTop(BORDER_THICK);
style.setBottomBorderColor(BLUE);

and apply the style on cell B3. Then B3 will lose the grey outline on its
left, right and bottom border, which is the default appearance in Excel. Is
this true and if so how can I set one border but preserve default looks on
other sides?

Thanks.

Zihao