Prevent nested table style leaks by using children combinators and CSS custom properties:

• Change in reboot: inherit border color and set border style to solid to make it easier to change borders. Border width is still 0, so no borders are visible by default.
• The only way to get the colors right is by only colorizing the td and th backgrounds, therefore we need to work with CSS custom properties.
• Stole the background image hack from @mdo in De-dupe .table-dark, add new border color utilities, and update table docs #30342
• All table variants are now stored in $table-variants. This way we have more control over colorizing tables. The dark table is also a variant now.
  • $table-border-factor will determine the contrast between the background and the borders
  • $table-accent-bg-factor will determine the contrast between the background and striped rows
  • $table-hover-bg-factor will determine the contrast between the background and hovered rows
  • The color of the text will be determined by contrast-color()
• Simplified the docs a little
  • Stored the table content in a partial, which is easier to reuse
  • Only kept the whole html for the first example, stripped some html in the other examples (inspired by the modal docs change)

TODO:

• Document BCs in migration guide
• Update partial template (see Prevent nested tables style leaks #30466 (review))
• The docs might need some adjustments
• Cleanup git if feedback Tweaks are applied
• Have a look at what tickets can be closed by this

Preview: https://deploy-preview-30466--twbs-bootstrap.netlify.com/docs/4.3/content/tables/

• Fixes Table child elements (thead, tbody, tfoot, tr, td, th) should inherit parent background color #29244, background issues are fixed
• Fixes prevents .table-striped to be automatically applied to inner tables #30251, fixes Nested table inherits classes that cannot be reset #27800, nested tables styles are reset
• Fixes Inconsistant .table-active color #24529, active is now a state and can be applied to any table variant
• Fixes .table-dark is used in two conflicting contexts #27879, table-dark is a table variant now
• Fixes Suggestion for .table-hover #28160, hoverable rows work on any table variant
• Fixes Table row-variants does not respect font-color #25675, correct colors are applied to the table variants
• Fixes Contextual table classes apply tr class instead of td class to individual cells #29290, table variant classes can be added to table,thead,tbody,tfoot,tr,th and td.
• Closes De-dupe .table-dark, add new border color utilities, and update table docs #30342