assistance with conditional formatting

Hi folks,

I need some help with conditional formatting.
I have a spreadsheet that shows an end elevation of a fan, it has 4 fan blades that sit diagonally. Depending on whether an option is selected will determine whether the diagonal is shown or not. I have the conditional formatting working for drawing box sections which works fine. My problem is that under the formatting border option within Conditional Formatting is greyed out. How can I get this option enabled?

I have attached a screen dump to show what I'm blethering on about.