You can't create variables in CSS right now. If you want this sort of functionality you will need to use a CSS preprocessor like SASS or LESS. Here are your styles as they would appear in SASS:
$Color1:#fff;
$Color2:#b00;
$Color3:#050;
h1 {
color:$Color1;
background:$Color2;
}
They also allow you to do other (awesome) things like nesting selectors:
#some-id {
color:red;
&:hover {
cursor:pointer;
}
}
This would compile to:
#some-id { color:red; }
#some-id:hover { cursor:pointer; }
Check out the official SASS tutorial for setup instructions and more on syntax/features. Personally I use a Visual Studio extension called Web Workbench by Mindscape for easy developing, there are a lot of plugins for other IDEs as well.
As of July/August 2014, Firefox has implemented the draft spec for CSS variables, here is the syntax:
:root {
--main-color: #06c;
--accent-color: #006;
}
/* The rest of the CSS file */
#foo h1 {
color: var(--main-color);
}