v4 table updates

[mdo]

Few table updates:

• Fixes the inverse table's border color. Adds a new variable that lightens the inverse table's background color a bit instead of using $body-bg.
• Fixes the responsive table behavior. Instead of using a parent class again, this scopes (as the comment above it indicates) the styles to under 768px.

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Fixes part of #21585
Fixes #21577 (/cc @pmsaue0 as credit for the fix)
Closes #21677, closes #21678, and closes #21676 as they're no longer needed.

[mdo]

Gave up on CI lol.