Acquia Cohesion renames to Site Studio from version 6.2. Tell me more.
The style preview may not be updated because the refresh icon hasn't been toggled to the active state.
You can style tables or forms through the use of base or custom styles.
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.