There are all kinds of ways to do it. A simple B4J only solution:
1. Add an invisible pane with the designer and set its background to GradientDrawable. Set the orientation to TOP_BOTTOM.
2.
B4X:
For Each col As B4XTableColumn In B4XTable1.VisibleColumns
Dim p As B4XView = col.CellsLayouts.Get(0)
CSSUtils.SetStyleProperty(p, "-fx-background-color", CSSUtils.GetStyleProperty(Pane1, "-fx-background-color")) 'Pane1 is the hidden panel.
Next