I notice people put warm clothes on idols in winter time. Is this a personal practise or is it supposed to be followed?
Vastra or clothing is one of the UpachAras (things that are offered for a deity's appeasement).
From this answer:
Atha ShodasopacharAh: Padyamarghyam TathAchamam SnAnam Vasana Bhusane |
GandhapushpadhupadipanaivedyAchaman Tatah ||
TAmbulamarchanA Stotram Tarpanamcha NamshkriyA |
Prajojayeccha PujAyamupachArAmsthu Shodasha ||
Padya (water for washing feet), Arghya (water for washing hands), AchAmaniya (water for sipping), SnAniya (water for head bath), Vasana (clothings), Bhusana (ornaments), Gandha (sandal, scents), Pushpa (flowers), Dhupa (incense), Dipa (lamp), Naivedya (eatables), Achamaniya, TAmbula (betel leaves and nuts), Stotram (hymns), Tarpana (act of offering water from Deva Tirtha in hands) and Namaskara (prostrations). A devotee should offer these 16 upacharas to God during puja.
So, Vastra or Vasana or clothing comes under the 16 necessary things that God is offered. During winter we ourselves wear warm clothes so offering God with the same is quite a normal thing to do for the devotee I guess.
The instruction that God should be offered clothes is a scriptural one but it simply mentions clothes and not warm or otherwise and that's because we are also given common sense to be used when the situation demands.
Rickross has provided the answer for formal worship. But even in everyday worships, we cover the deities by warm cloth in winter and fan them in summer. This is because worshipping means adoration. The devotee feels that God also suffers from heat and cold, hunger and thirst and serves Him or Her accordingly. This is because of love of God and these are called sevas.Even if no pranaa-pratistha is formally done, such clothes are given. As Sri Chaitanya Charitamrita puts it:
"Krishna-indriya-peeti ichchha dhare prem naam"( Love of God menas serving the organs of body of God)
He also says:
Aapanara baro bhave, amaare sama, heen/sarvabhave aami hoi taahar adhin(I am always bound by someone who thinks oneself equal to or subordinate to me).
This feeling of equality or compassion is the cause of giving such services like clothes.