The One Click comes with some predefined styles that you can choose from.
To choose the style you wish to use, you need to access the Theme Options by going into:

VANILLA BACKOFFICE > APPEARANCE > THEMES OPTIONS

and change the Current Style to the desired one and save it.

After that, you may want to make some adjustments on the style, so you can use the Style Editor.
Also, if you want to change the disposition of the elements of the theme you can use the Layout Selector.

When you are satisfied with your style you can go to Settings to make the last configurations.