Adaptive theme css which stylesheet to use?
The AdaptiveTheme site has a lot of great documentation to help explain this.

In general, you should use global.styles.scss to style elements that will look the same across all devices, and the individual responsive stylesheets (e.g. responsive.desktop.scss) for device-specific styles. If you want to override something in global.base.scss, you're probably best off doing it in global.styles.scss, depending on how fundamental the change is.