Hamalrad, you can customize the title and the body of the welcome email, but the text on the action button is not customizable right now. However, the color of the action button will be the same color that you have customized for your community.
Maybe other people want this feature to be added to our product. Don't you mind adding this to the request feature page here: https://community.tribe.so/question/5b88379890ecbe6751123d8b63
There we can see what others thing about it :)