Use a monospace font for countdown

**Is your feature request related to a problem? Please describe.**
The digits "jump around" when changing as we're currently using a variable-width font for them. This is especially annoying as due to the centered text, whenever a digit reaches `1` (i.e., at least every 10 seconds), the second digits jump horizontally by a few pixels.

**Describe the solution you'd like**
Use a monospaced font for the digits

**Describe alternatives you've considered**
One alternative might be to align the digits inside the boxes to the left (still somewhat centered, but text-aligned left) to avoid so much jumping, but that's far from ideal.

**Additional context**
GIF: https://i.imgur.com/2t5eBYH.gifv