In the documentation says "Mibew Messenger automatically scans the styles directory, so new chat styles becomes available in the button generation form and at the 'Settings' page immediately after addition.", it's not work!
1. There are no known problems with style management in Mibew Messenger 3.2.4. It just works.
2. If something doesn't work for you, you're doing something wrong.
3. You hadn't provided any details, so it's impossible to tell what exactly you're doing wrong.
What should I do???
You can start with providing all details that could be useful to give you a meaningful answer. What is your environment (hosting details, software versions, etc.)? What
exactly are you doing? (Step by step, with all details). What are you expecting to see? What are you actually see? What's in the error log of your web server?
With full and honest answers to that questions maybe it will be possible to help you. Otherwise you're on your own.