Support configurable `body` color

[markdumay]

Problem or enhancement idea

Hinodes supports Bootstrap's eight theme colors. The body color is not configurable though.

Proposed solution

Extend the configurable colors with body. The variable is applied to the page background (considering the dark/light theme) and is available as Sass variable.

Alternatives

The colors can be assigned using regular stylesheets too, however, these are less easy to manage.