Ubuntu mail notification (envelope icon) does not change colour..

Mail notification icon (envelope) change colour on its own when it wants to. Sometime it turns blue or green but most of the times it doesn't give a hint that there's a new unread mail.

I've read few old posts and come to know that it actually is some kind of (irritating) feature that only shows notification (changes colour) for starred or important mails.

When I click the envelope icon it shows the mailbox name and number of unread email but it just doesn't change colour that I feel just defeats the purpose of a notification tool.

I wanted to know if there's any option (there must be some) to make it change colour for all mail types and to customise what colour to what mailbox or type of message (like red for important messages, orange for normal?).

I'm using Thunderbird 38.5.1 on Ubuntu 15.10 system.