How i assign the same style to all the buttons in my application??
By default they all have the same style.
You're probably talking about that annoying "Default Button" that changes one button on each page :)

Good news: you can just manually change that one button on each screen that is the default(y) one.
Bad news: that button shouldn't go away. The pressing of Enter key must call something. You can:
-embrace that condition in you UI
-you can create a dummy invisible button on each screen to do nothing.
- or you can manipulate CSS and overwrite the button style. Like this:

.Button {
    background-color:  #AAAAAA !important;
    border-color: #BBBBBB ! important;
Thank you Nuno reis.

But if I want to change the style? can I assign an id to all the buttons and define the style to the id?
You don't need to assign an id. They already have a class. You can edit the .Button to get what you wish.
Thank You.