I think you're making it more complicated than it needs to be.
1. Select all the cells you want, Right Click > Format Cells
2. Go to the Fill tab, select the color. You'll see all your cells look like one block now.
3. Select smaller selection of cells inside. Right Click > Format Cells
4. Go to the Border tab, select the 'Outline' Preset.
That should do it for you. It'll look like so: