Fix default highlighting in custom color schemes (#986)

* Fix default highlighting in custom schemes

Fix #982

The variable settings for highlighting in the default `light` scheme are currently (v0.4.0.rc2`) in `_sass/color_schemes/light.scss`.
This PR ensures that custom schemes are based on the `light` scheme.

It also adds a note explaining the default to the customization docs,
and gives an example of how to define a custom scheme based on the `dark` scheme

* Prettier

* Deleted test file

Co-authored-by: Matt Wang <matt@matthewwang.me>