Fides Admin uses colors & styles from both the default theme color schemes and the included core color helpers.
The core color helpers can be applied to all kind of elements from buttons to icons, headers, headings, etc. They do not change when you switch to a different theme from the same template.
These are the template default color schemes and are used site wide to consistently style common elements. For the primary color you must add .primary-bg and for the secondary color you must add .ui-state-default. Note that theme two styles change based on what color scheme you choose for your template.