Separate base theme into own package

[jdhuntington]

In Fabric 6, all consumers automatically import the default theme out-of-the-box. This is great for casual use but becomes an encumbrance once optimized size becomes a project goal.

Furthermore, most custom themes presently depend on the base theme that ships with Fabric. This has potential unfortunate side effects of changing the appearance of custom themes if design alterations are made in the base theme.

This feature covers separating the base theme into another package. It will be included in the default Fabric package, but it can be omitted by requiring an "unthemed" oufr-package instead. In conjunction, a separate theme base allows custom themes to depend on a specific version that will not be affected when upgrading the Fabric core.