Okay look for this block in the default.php file.

.ossn-button-green,

ossn-login input[type='submit'],

ossn-home-signup input[type='submit']{

box-shadow: 0px 1px 1px #F55214 inset;
border-color: #F55214;
background: linear-gradient(#F55214, #F55214) repeat scroll 0% 0% #F55214;
color: #fff;
cursor: pointer;

Myself I have already changed the colors so mine will look different than what is in your file. But now do you see where it say's #F55214 that is the code for the color I am using you will see two different set of numbers in yours the reason for that is when you click on the button on yours you will notice it changes color...Mine doesn't.

Now how do you get the codes...well since there are thousands of them the easiest way is like I did it in the google search bar just type in

"The color you want" color code

So this is what I did.

orange color code or you can type in color code for orange.

I hope this helps you.

P.S. If you want to change all of the buttons and background colors like I did you have to g through all of the files and find the number codes and change them to the color code you want...Be careful and make backups of the files you do change just in case.

I cannot change all the buttons in it...?

[DUPLICATE OF : http://community.opensource-socialnetwork.org/discussion/view/479/changing-the-colour-of-a-button]