This information is also stored in the /assets/img/clockface README.md

Clockfaces are generated with: http://www.oliverboorman.biz/projects/tools/clocks.php

Settings are as follows:

• Colour Format: Hex
• Canvas
  • Scale (%): 100
  • Background Colour: transparent
• Clock Face
  • Back: transparent
  • Edge: transparent
  • Tick: #4d4d4d (keep slider at default, but it is 5 from the left)
  • Subtick: transparent
• Numerals
  • Colour: transparent
  • All other settings @ default
• Hands
  • Hours
    • Colour: #4d4d4d
    • All other settings @ default
  • Minutes
    • Colour: #4d4d4d
    • All other settings @ default
  • Seconds
    • Colour: Transparent
    • All other settings @ default

Then download as PNG and name the file "HH.MM.png" e.g. "11:35.png".

Note: For transparent, click on the color selection box (where you would enter the hex code) and drag the opacity slider all the way down.