Change to Appearance > Grid

This is a breakpoint specific appearance page. Mind the Viewport Switch at the top of the settings screen.
All values you enter or edit while a certain breakpoint is selected will affect the frontend appearance of your WordPress page for this breakpoint.

By clicking on any of the breakpoint titles you will notice how the appearance page itself changes to represent the corresponding values.

When selecting anything but the first breakpoint you might see small labels inside the settings fields which inform you, that those values are falling back to values entered underneath a smaller breakpoint (sm in the displayed case) …

… Switch to the relevant breakpoint to see the values itself.

Use the Reset All Elements button to reset each and every value for the current breakpoint.

By selecting any breakpoint from the Copy values from … dropdown you are able to copy those values to the currently selected breakpoint.

Let's change some values …

… while doing so you might notice some values with a label and a small note right behind them.

Those values are multiples of the labeled value.

So in this example the top and bottom Page Paddings depend on the originally set Line-height value. This is important to keep every element aligned to a grid.

Let's change to a larger Viewport and enter some values that differ from the ones we entered before.

When activating an even larger Viewport notice how the label informing you about which value is going to be used changed from SM to MD because you changed some values in the Medium breakpoint.

The Content Width value let's us define a maximum page width.

Always confirm your changes by clicking the Save Changes button in the upper right corner or on the bottom of the settings page.

Now when clicking through those breakpoints you notice how the individual settings change its values …

… this happens on the frontend as well. Users with a device using the Small breakpoint will have those values activated, user with a device qualifying for the Medium breakpoint those values and so on.