feat(tables): Rounded corner tables
feat(tables): Rounded corner tables

Because rounding corners with a wrapping panel and overflow: hidden causes drop-down menus to not be able to cover a table, we have to implement rounded tables without clipping.

This moves the background onto the cells of th and td within thead and tfoot, and creates a class for the header color style, orthogonal to the rounding.

The borders are applied to the corner cells themselves, since tables and thead/tbody/tfoot don't contain their children in the traditional layout sense.
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