Page customizations are similar to Block template customizations, except instead of being block by block, they are page by page. Page customizations are also employed on a per-URL basis, meaning that changes will follow the inheritance principle. In addition, several URLs will have customizations that are specific to that URL (Classifieds, Users, etc.).
In the event that you want to preview an ad that you have placed on your site, as opposed to the grey Ad information, you will need to use the debug tool. This will work to show previews of both ads in fixed positions and those that are block-based.
The CSS class field can be used to make a block visible or hidden based on a device's view size (desktop, tablet, phone). This is useful for large blocks (like a slideshow, a leaderboard ad block, etc.) where they won't fit width-wise on a phone, so you can use a CSS class to make the block visible on large sizes, or hide the block on small sizes.
Learn how Utility Regions can be used to place content blocks within assets, section and search results pages.
Learn how presentation modes can be used to change the look and feel of article, image and collection assets.