Setting Desktop Themes
Telegram Support1: First, you need to find a theme file or a link to the theme you like. This telegram channel has many themes already.

2: Click on the link or the .tdesktop-theme file, and a preview will open. To see how this theme looks in the app, click "Apply this Theme" at the bottom right.

3: If you like the theme, click "Keep Changes" within 15 seconds.

4: 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.