❓ HOW TO GENERATE A THEME

To create a theme with a background image included you should put two files in a .zip archive:

1⃣ First one is the color scheme:
'colors.tdesktop-theme'.

2⃣ Second one should be the background image:
It can be named 'background.jpg', 'background.png', 'tiled.jpg' or 'tiled.png'.

Tip: You should name it 'background' (if you'd like it not to be tiled), or it can be named 'tiled' (if you'd like it to be tiled).

▶️After that you need to change the extension of your .zip archive to 'tdesktop-theme', so you'll have:
mytheme.tdesktop-theme
|-colors.tdesktop-theme
|-background.jpg (or tiled.jpg, background.png, tiled.png)

🌐 Sample theme:
https://github.com/telegramdesktop/tdesktop/blob/dev/Telegram/Resources/default.tdesktop-theme#L1