How do I see the style entries that make up the mixins without applying them to my theme? For instance, how do I know what a fancy-button looks like?
Mixins are one of the most powerful Sass features. They allow re-use of styles – properties or even selectors – without having to copy and paste them or move them into a non-semantic class.
The mixins used in NationBuilder themes (gradient, border radius, box-sizing) come from Compass. You can learn more about them here: http://compass-style.org
'Fancy Buttons' is the only non-Compass mixin that has been added to NationBuilder. All the submit buttons use them by default. You can learn more about them here: http://brandonmathis.com/projects/fancy-buttons/
To use your own mixins in place of existing ones, you can just remove the @import and customize your own styles.