The Inspector (right sidebar) is where you edit the styles of your elements, and where you can find advanced settings to control how your elements behave. Finally, it contains the Code tab, for inspecting and exporting the live code of your designs.
The Styles tab is where you control all the various visual properties of your elements. The controls inside the Styles tab are used to change how the currently selected element looks, within the selected Breakpoint (such as Phone or Desktop) and within the selected State (Such as Default, Hover).
Each style controls has the name shown on the left. This name lights up in blue when the style is set.
To unset it, click the label, and click Reset (Alt + Click).
When the label lights up yellow, it means that the value was set in a different breakpoint or state.
Example: You have an element that you want to have a 50% width on Phone Portrait. When you switch to Desktop breakpoint, it will show the Width property in yellow, because Desktop styles are inherited from Phone portrait. When you set the width to 100% on Desktop, it will light up blue again because you've just overriden the value that came from Phone Portrait.
Another example: You have a Button element that has a red background. You want the background to be dark red when pressing the button. When you switch the State to 'Press'. The fill property will light up yellow, because it was defined in the default state.
Read more about how responsive styles work.