Add print background colour and cell border for Firefox (#1993)

## What are you changing?

- Add a print-only background colour to each cell
- Add a print-only stroke colour to each cell 

## Why?

By default, Firefox doesn't support printing background colours. This
behaviour must be [enabled using a setting in the print
dialog](https://superuser.com/questions/1113516/is-it-possible-to-view-a-page-in-firefox-using-print-media-type).

We can get around it using the `fill` property, which is currently set
to `transparent` for the black cells. We can use the `stroke` property
to set the border colour between cells.

## Screenshots

**Before**

<img width="366" alt="Screenshot 2025-03-05 at 17 57 25"
src="https://github.com/user-attachments/assets/6f2cbb76-0499-4014-b1db-28037ce658d7"
/>

**After**
<img width="368" alt="Screenshot 2025-03-05 at 21 25 29"
src="https://github.com/user-attachments/assets/dcbb2320-a1b1-4afb-bd6c-afdbdf6b6f1d"
/>

Author: SiAdcock

libs/@guardian/react-crossword/src/components/Cell.tsx

@@ -65,7 +65,10 @@ const CellComponent = ({
 	const cellStyles = css`
 		fill: ${backgroundColor};
 		@media print {
-			fill: ${isBlackCell ? 'transparent' : theme.gridForegroundColor};
+			fill: ${isBlackCell
+				? theme.gridPrintBackgroundColor
+				: theme.gridForegroundColor};
+			stroke: ${theme.gridPrintBackgroundColor};
 		}
 	`;