In order to change the style of all buttons, you need to first define what HTML code is used to represent buttons on your website.
Usually the following tags are used to create buttons:
So you will need to create multiple styles to cover multiple different tags:
Buttons using tag
Note that tag is usually used as a link so in order to make it look like a button, you should add it some class:
When you have all link buttons marked by this class, you can simply define the button class using the following selector: