Is Bhikkhu allowed to keep his food in refrigerator for next day or for long period?
Not proper food, only certain types of edibles:
The Four Sorts of Edibles
Any nutriment that a bhikkhu puts into his mouth is classified in four
groups, which specify the time limits during which he can consume or
(i) Food — Limited from Dawn to Noon (Yaavakaalika)
(ii) Fruit juices — Limited to One Day (Yaamakaalika)
(iii) Medicinal-tonics — Limited to Seven Days (Sattaahakaalika)
(iv) Other Medicines — For All One's Life (Yaavajiivika)
-The Bhikkhus' Rules: A Guide for Laypeople
This is also what I've seen in some western monasteries.
Is this common sight to see a refrigerator in a monk's room in Theravada monastery?
The few times I've been in a monk's private room, I never saw one. But in the same housing (under the same roof), it's common.