windowBackgroundWhite sets the background almost everywhere except the chat screen background. For example, it sets the background on the chats list screen or the background of sections in Settings. This variable also sets the bottom panel in the in-app player.
windowBackgroundGray sets the secondary background like in Settings between sections or below calls log if it doesn't fill the screen fully.
chat_wallpaper sets the background on the chat screen, accepts either an image or a color. You must change this variable so Telegram includes your wallpaper in your theme. Changing the wallpaper in Settings → Chat Wallpaper won't include it in your theme. Be careful: if you set an image as a wallpaper, Telegram will compress it every time you change any variable with the in-app editor. For this reason, you must set your image again before you share your theme.