Question: Due to impermanance of everything, Are there anything to be happy or sad about ? Even about **dhamma** ?

If something seems like to be happy, there are some reasons to not to be happy.

1. It is impermanant for sure. but also,
2. That **happiness make sadness in the future (future includes next lives)** (I need to be sure about this sententce, please mention about this in your answer)

Are enlightened people happy ? Or neither happy nor sad ?

Answer: 
I think Buddhism recommends you focus on what's virtuous and moral instead of on what's feels pleasurable.

For example, lying, killing, stealing -- are all to be avoided -- and that's important.

Conversely, generosity, kindness, keeping your promises -- that's important too.

There is a word -- [sukha](https://en.wikipedia.org/wiki/Sukha) -- which might be translated "happiness". It's used in contexts like the [Sukhavagga](https://www.accesstoinsight.org/tipitaka/kn/dhp/dhp.15.budd.html).

I suspect it's a happiness conditioned or caused by an absence not by a presence -- for example not happiness because you have something ("a chocolate bar") but happiness because of not having something ("anger", "greed", "remorse").
You can enjoy looking at a lovely flower, even though you know it will die at some point. The colour and scent are not diminished by the fact it only lasts a few weeks/months.