bug(web): Keyboard error feedback (beep) poorly designed for dark modes

> This is probably not the time for this, but of course if we are in 'dark mode', the beep will not be very visible!

_Originally posted by @mcdurdin in https://github.com/keymanapp/keyman/pull/2830_

Basically, for 'native'-mode KMW, we give error feedback by 'flashing' an element momentarily - and we currently use black for that. Black-on-black for dark modes isn't likely going to have the contrast needed for this purpose.