Select the cell with the style you want to copy and click the right mouse button: select the Style command from Cell Style | Copy in the popup menu. Now select the second cell, open the popup menu again and now select the Style command from Cell Style | Paste.
Remember that these commands have no effect on empty cells.
Alternatively, you can open the Cell Style window, go to the Library section and click on the Add button: in this way you add the style to the library and it can be applied to other cells that are either in the same or in different projects. Cell styles can also be imported/exported as files.