In Spanish, "bonito" is an adjective and does not have a "they" conjugation. Instead, it agrees in gender and number with the noun it describes.
If you're referring to the plural form, "bonito" can change to "bonitos" when describing a masculine plural noun (e.g., "los cuadros bonitos"). If describing a feminine plural noun, it changes to "bonitas" (e.g., "las flores bonitas").
So, "they" would not change the word "bonito" itself, but rather it would be used in the context of what is being described.