Acquia Cohesion renames to Site Studio from version 6.2. Tell me more.
Base styles are CSS styles for HTML elements.
No, you cannot layout forms with Acquia Cohesion.
You can create more than one template to provide layout and style alternatives with the use of the template selector.
You can style tables or forms through the use of base or custom styles.
No, the entity browser does not work for component defaults as the entities are content, not config.
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.