Happeo pages has numerous possibilities for styling. You can make your pages look anyway you want, which means that styling a Page Group requires a bit of learning.
Start by opening edit mode and going to the Page group styles -section. From here go to Styles and then "Edit my styles".
Here you can see the main sections of styles:
When you are happy with your styles, press "Save styles". If you want to remove all styles, click the "Clear styles" which will revert the page group to the default settings.
Styles apply to the whole page group.