Created attachment 116253 [details]
Screenshot of table border properties tab

1. New Impress document
2. Insert table, any size
3. Right-click table object, pick Table... from context menu
4. Click Borders tab, open Line -> Style listbox

Observe previews of white lines on white background i.e. almost nothing to be seen (as in attached screenshot)

It is the case with all predefined table designs (sidebar -> Properties -> Table Design panel) because their default table border colour is set to white. Maybe the real background of table cells could be used in previews instead of hardcoded white ...

It's been like this since 4.3 at least, so likely not a regression