But should we modernise it?
5000 years ago there was no such thing as running warm water or TV.
Where do they get the idea that electricity is unkosher?
Who's "we?" Are you Jewish?
No one said electricity is "unkosher." During the weak it is laudable to use electricity to learn Torah, carry out daily activities etc. No problem. On Shabbat, it is forbidden to use it based on what constitutes a forbidden behavior in Shabbat. A certain type of activity is forbidden on Shabbat for Jews and always has been. So the question is, whenever a new activity comes out, does it fit with those criteria that cause something to be forbidden to use on Shabbat, or does it not? It was a complicated discussion when it came out to determine whether it fit certain criteria. They consulted experts in physics and electricity itself to gather the proper information to be able to decide. Nothing ancient or archaic about it.
So it is not forbidden if the electric device (say, a lightbulb) was already on before Shabbat and left on for the duration of Shabbat. To shut the device or turn it on would be to do the forbidden activity. That is why Shabbat observing Jews might leave the bathroom light on all Shabbat (they turn it on before Shabbat starts), so that people can see when they use the bathroom. Nothing "unkosher" about electricity in of itself.