Need help with php/css

I am very green when it comes to php and CSS and I would appreciate some general help.

I have a website that I have duplicated/migrated and had some php/css work done in the past. I would like to replicate this work on the new website.

This is what I need to know:

I have a page that has the following element in it:

<div class="secure-order-form">

The styling for this is found in gstyle.css, and inside this file is this along some other styling for other areas of the same web page:

h6 {
color: #1A1385;
font-size: 24px;
margin: 15px 0 25px 0;
text-align: center;
}

.secure-order-form {
display: block !important;
background: transparent url('http://www.gallbladderdetox.com/wp-content/themes/OptimizePress/images/special/secure-order-form-background.png') repeat-y;
margin: 0 auto;
width: 864px;
}

What I am trying to understand is how wordpress knows the styling for "secure-order-form" is found in the stylesheet "gstyle.css"

How does it know? :confused:

I would like to replicate this if possible.

Any help and advise would be appreciate it.

Hey there,
So on any wordpress theme, ( or any website really. )
you should find something like this

<link rel="stylesheet" type="text/css" media="screen, print" href="style.css" />

It pretty much attaches your CSS to the page.

Im assuming this is what your talking about.

Thank you very much for this answer. I really appreciate it. This is what I need it.

I believe that that is what I was searching for. On which file would you find this. There are so many of them.

Thank you again:)