WhatsApp has several default themes that you can choose from to personalize your chat experience, but unfortunately, you cannot customize the themes in WhatsApp.

However, you can customize some aspects of the appearance of WhatsApp, such as the chat background and the chat bubbles. Here's how:

Change the chat background: You can change the background of your chat by going to FM Whatsapp APK Download Settings Chats Chat Wallpaper. From there, you can choose from the pre-installed wallpapers, or use one of your own photos.

Change the chat bubble color: To change the color of the chat bubbles, go to WhatsApp Settings Chats Chat Bubble. From there, you can choose from several different colors for both incoming and outgoing messages.

Change the font size: If you find the default font size too small or too big, you can adjust it by going to WhatsApp Settings Chats Font Size. From there, you can choose from several different font sizes.