By clicking on the cogwheel on your Power Bar (on the left-hand side), you can easily customize the design of your Frontify Guidelines.
You have several options to do so. The list below is non-comprehensive but should give you some inspiration on where to start:
Navigation; Change the number of navigation points you show and how they are displayed
Header: Customize the color, background image, and size of your header
Content: Customize the color, line height, size, and much more for the text areas in your Guideline. You can change those settings per text type (H1, H2, ..., Quote)
Accent Colors: Define the color you want to be displayed in content blocks, such as the callout block
Buttons: Customize your buttons' default and hover states. You can define colors, background colors, borders, and many other things.
Just jump into it - customization of Guideline couldn't be easier.