Make the Var component's input box more obvious

[ptgott]

Users are missing the fact that Var components are input boxes. We should employ UX best practices to make it clearer that we expect users to fill them in.

At the time of opening this issue, the Var component looks like this:

There is low contract between the text of the Var component and the background of the code snippet, with only an understated pencil icon to indicate that the component is editable.