To know more about Telegram I send you to their website and let you read what telegram is about : 

https://telegram.org

Installing could not be simpler.

Then you start the application and make sure you are able to use it using your smartphone number.

But this article is NOT about how to use telegram. That does not need a tutorial really.

This article is about the blue icon in our toolbar.

It breaks our theme and we want to see a white icon there.

Get the scripts from the arcolinux-nemesis github : 

https://github.com/erikdubois/arcolinux-nemesis

And run the script in the Personal folder concerning telegram and your telegram icon will be white again.