On the left hand side of the Power Style Editor, you will see groups of settings. If you expand the settings groups, you will find a set of settings that applies to that group.
You can decide to only over-ride some of the settings ad keep the default for others. For example in the Quick Start Guide, we only changed the background color.
To over-ride a style for a particular setting, you would change the drop-down for that style to something either than "Default".
Check the settings reference for the specific settings group for more details on the options.