Reset timestamp faces when theme changes

The automatic generation of faces for the time column is pretty sweet, but if I toggle between light and dark themes, the faces don't get updated. I could presumably add something to `modus-themes-after-load-theme-hook` in my case, but I'd need a function to call which can do that.

For now the workaround is closing the rooms list buffer and reopening it, so it's not high priority, but I figured I'd put the request in.