The desktop styles are the default styles for your content, meaning that if you set a style on desktop it will be the default style for tablet and mobile unless you set a specific tablet or mobile style. If you set a style on desktop and do not want it to be applied for tablet or mobile, then you should switch to the tablet size and set that style how you want it to show on tablet and mobile. For most designs you should not need to make drastic changes between devices (in practice most changes will be padding, margin, etc).
If you find yourself making the mobile version very different than desktop, it might be easier to break the different designs into whole different sections and hide/show the entire section based on device size. This forum post talks about how to do that, as well as some other great tips: Mobile styles affecting desktop styles