Use a heat pad and a seed germination tray.

Or cover a cup with clear plastic wrap over the top to hold in the humidity. It's important to keep tiny citrus plants from seed warm and moist.

Good luck with your seedlings!

I do have a small LED lamp designed for growing herbs indoors(Tregren Genie), 5,5w and 800lumens which I have 5cm above my seedlings. It keeps them warm and I lightly mist two times per day.

They also receive light from my two bigger CFLs because they are all an the same table. What comes to light and warmth, they are not the issue, the biggest and most difficult thing to keep in check is the hudminity. During the coldest temps in mid-winter the air gets pretty dry.

Last winter I kept the hudminity higher by putting the clothes rack in the plantroom while drying washed clothes. This winter we've moved to new house and I don't think my wife lets me do it anymore, maybe the solution is hudminity trays..

The most important thing for seedlings is watering from the bottom to avoid damp off. Humidity is not important except for keeping the seeds moist during germination. Seeds do not need light to germinate; but after sprouting more light leads to a shorter stronger plant.