Acquia Cohesion renames to Site Studio from version 6.2. Tell me more.
You have to upload a JSON file to provide the index of the icons within your chosen icon font.
You can use Elements to create content but we don't recommend doing so for multiple reasons.
The Youtube video embed is responsive so it should expand to the width of its parent. If you do need to set a width you can apply the Width CSS property through the Style Builder.
Custom styles are CSS styles for classes.