Setting Telegram Android Themes
Telegram Support1: First, you need to find a theme file that you like. This telegram channel has many themes already.
2: Tap on any .attheme file and if you like it, tap on 'Apply' at the bottom right of screen.
3: Tada!
See also:
To contact Telegram support, go to Settings, and scroll down until you see the 'Ask a Question' button at the bottom.
If you want to join us, check out the Telegram Support Initiative.