You can try playing with Format > Option Inspector > Cell Options > Display Options > CellMargins.

You may need to have your cells selected first.

Thanks, Hans Milton. I didn't mean to join the cells. But your answer is also useful.

Thanks, it helped a lot. I didn't use exactly the way you suggested because although I found the CellMargins option I couldn't change via Option Inspector. But since I knew (from you) the existence of CellMargins I used it while defining Styleshhets OR Show Expression of a cell and adding the definition CellMargins.