Practically everything can be modified if you're willing to learn about userChrome.css, but for a simple way to restore colored icons, try Phoenity Icons.

https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

Yes it is very helpful. Thank you.

Now it is much better. It`s shame that there is no choice during update. Previous look last few years and I`m like old tree... ;)

I will also dig into userChrome.css just because my curiosity.

I applied the Phoenity Icons as well. Although a better look than the ones delivered with the latest up date I don't understand the constant need to change things that users like. These mindless modifications do nothing but make users frustrated. Improvements in functionality, security and ease of use are always welcome but leave the look alone or at the very least give us the ability to opt out .

The Phoenity icons help greatly. Thanks.

But this should not be necessary. It is a pain to go looking for a solution every time a major upgrade wipes out our previous solution.

Given the very large number of people who consider the default "modern, minimalist look" to be butt ugly, why can't Thunderbird offer the more classic look as a built-in theme. If both dark and light themes can be offered, a classic theme doesn't seem like too much to ask.

You can bring your concerns to the attention of the UX developers in this forum:

https://thunderbird.topicbox.com/groups/ux

Some options for 'classic' folder icons are discussed here:

https://support.mozilla.org/en-US/questions/1311513#answer-1363777