Now that you've got some Slate Styles created, let's apply one to a Chart on a Dashboard.
From the Slate Editor of any Dashboard, click on a Chart that is on your Slate. Once a Chart is selected, you'll see four icons appear in the top left toolbar.
Click on the Slate Styles icon:
This will bring up a modal that allows you to add styles, reorder the styles, and even apply conditions to a particular Chart.