How to create my version of client mumble ?

[alkane]

I have seen some variations of themes for mumble but I would like to create my own version, where can I find the open source client version to edit it?

[Zing]

I'm not entirely certain on what you are asking. If you want to create a theme here is the Wiki article: https://wiki.mumble.info/wiki/Themes
If you are trying to create your own entirely custom client than here is the source code: https://github.com/mumble-voip/mumble

Hope this helps.
- Zing