If you create a table in KompoZer and then try to select a group of cells within it in order to apply some formatting to it, you will find all your formatting requests are ignored. This is a bug!
A trick that works is to set up the table with one row and as many columns across as needed. Then set the properties as you enter the contents of each cell. When you're done with the last cell, hit the tab key. The next row will appear and all the cells will have the same properties as the cell in the row directly above.
NOTE: This only works when you add a last row to a table. Inserting a row above or below will pick up the default properties.