Styles are universal. Any changes you make to a style will also change any other layouts that might be sharing that style. You will need to create a new style and name it specifically for the layout.
Any style can be copied by first opening the style, clicking on the properties icon, closing the style properties and then clicking the ‘Create New’ button. This will copy all the information from the last style you had open.
Change the description of the style so that it is easy to locate from the list. If the style is unique to a layout, you could include the name of the layout so that you know this style will be unique to this layout: