Question

if a cell has a dark border around it, it means

Answer 1

The dark border of the cell shows that it is selected and active.

Answer 2

If a cell has a dark border around it, it means the cell is selected or the cursor is the particular cell.

Explanation:

All the other remains normal with the grey borders and the cell when selected it gets the focus and the border of the selected cell changes from grey to dark.

The data form of the cell is symbolic representation to say the user that the particular cell is in selected mode in the given document.