Hi Eric,
" …what I was asking about is how to make the highlighting around the currently selected cell, which is B10 in your example, more bold so it's easier to see."
I was aware of that. B10 shows the automatic highlighting of the selected cell(s) that is built in to Numbers. That highlighting produces a thin blue border on the selected cell, but does not change the weight of the text in that cell.
Cells C3 and E6 show two types of highlighting that are not available via automatic processes, but can be applied manually to selected cell(s) by the user. The first is a heavier cell border, more visible than the one applied automatically by the application; then second, in cell E6 is a background fill in a brighter colour, also applied manually.
Either of these highlights can be applied with small effort on the part of the user, and can be removed by equally simple actions. Their disadvantage, of course, is that they require user actions beyond simply selecting the cell to apply the highlighting and to remove the highlighting when it is no longer needed.
Regards,
Barry