Add missing `_color.less` variables to schemas

### Subject of the issue
The following variables are present within the theme but missing from the theme schema.

* `@body-background-color`
* `@drawer-item-selected-underline`
* `@pull-quote`
* `@pull-quote-inverted`
* `@pull-quote-border`
* `@shadow-opacity`

### Expected behaviour
The schema files are updated to support these variables.
