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

2: Click on any .tdesktop-theme and 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.
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