If I want to change(visual, UX, and place) just one button, Ex:
Should I create another template/theme? Or it could be done with plugin?
There is the concept of themes which is similar to plugins. But why stop at just one button?
@Nicolas For small one-off changes like this, consider using the "Custom CSS" screen in the ACP. This doesn't help if you want to move the button around, so if you're looking for more in-depth changes, you'll have to consider creating your own theme.