This is a simple browser clock that works on old smartphones to turn them into clocks. It uses the internal clock time, and so has all the time features of the phone. It does not have any other features such as colour change, alarm, or 24h clock. You can easily clone this project and make the changes you want, though.
To use, just point your browser to https://dneuman.github.io/clock/index.html. It will only download once, so you don't need to keep wifi working, though this may reduce the smartphone clock accuracy. Adjust the brightness on your phone to set the level you want.