Icons with monochrome colors and minimalistic style for the replacement of the standard coloured icons of Alarm Clock applet.
Make a backup of the original icons, located in "/usr/share/icons/hicolor/22x22/apps/" just in case. Archive contains two icons for normal and triggered state for both themes. Extract them somewhere. Pick the ones in DarkTheme folder if you use Ambiance theme + ubuntu-mono-dark icon set or the ones in LightTheme folder if you use Radiance + ubuntu-mono-light icon set.
Replace "alarm-clock.png" and "alarm-clock-triggered.png" in "/usr/share/icons/hicolor/22x22/apps/" with the selected icons from the archive (you will need "sudo" for this operation).
Relogin to make the changes take effect.