Conditional Formatting in Word - strikethrough

Rather than applying direct formatting to each individual cell that needs this format, define a character style that applies both the grey color and the strikethrough. Then you can apply that style to the text of any cell with one click.

To set this up, open the Styles pane by pressing Ctrl+Alt+Shift+S, and click the New Style button (the leftmost button at the bottom of the pane). This opens the Modify Style dialog. Enter a name for the style and change the style type from Paragraph to Character. Click the "New documents based on this template" option at the bottom.

Next, click the Format button at the bottom left and click Font in the menu. In the Font dialog, choose the shade of grey in the Font Color box, and check the box for Strikethrough, leaving everything else unchanged.

Click OK in both dialogs. The new style will appear in the Styles gallery on the Home ribbon.

To use the style, just select the cell(s) of the completed task(s) and click the style in the gallery.

You can assign a shortcut.

Go to File > Options > Customize Ribbon and click the Customize button at the bottom left. In the Customize Keyboard dialog, scroll to the bottom of the Categories list and click on Styles. In the list on the right, click the name you assigned to the style.

In the "Press new shortcut key" box, press the shortcut you want to assign to the style. Here I've shown the result of pressing the Alt, Shift, and G keys together; you can select pretty much any combination you like. If the line below the Current Keys box shows "(unassigned)", that's a good shortcut that doesn't already have a function. If it shows something else, you can assign that shortcut to the style and it will be removed from whatever its previous assignment was.

To make the shortcut stick, you must click the Assign button before closing the dialog. That will cause the shortcut to appear in the Current Keys box.