Allow user to specify background colors for layouts.

Add readme from updated implementation.

The information on how to use the library is indicated in the Usage
section of the README file. In addition, the copy text on the
Configuration section has been corrected.

The CSS variables have been documented to help with guiding the user
for configuration.

The components have been adjusted to define variables internally. They
will use internal defaults but will inherit their ascendant's CSS
variables.

Add files from TSDX.