Appearance

Go to  "Settings" → "Appearance".

Chats Design

To change the background of chats, click on "Chats Wallpaper" and select an image or upload your own.

Select the font size for messages.

Turn on bold if necessary.

Turn on the compact view of the chat list to hide message previews.

Themes 

Themes are divided into two types: dark and light. All dark themes are signed - "Dark ...".

The dark theme repaints the application in dark shades. It is suitable when you use the app at night.