Acquia Cohesion renames to Site Studio from version 6.2. Tell me more.
We recommend using decimals e.g. 1.3 as this makes the line height relate to your font size.
You don't. You apply a Descendant selector. These use a (Space).
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.