Fix markdown table row background and borders.

[isoos]

• Fixes Code blocks are indistinguishable on alternate table rows #6981
• Instead of using yet another color for light and dark theme, the table can get away with gray+transparent overlay colors, used for the border and the background.

Light theme before/after:

Dark theme before/after:

(The screenshots don't do much justice to it, will push to staging).

[isoos]

/cc @parlough

[parlough]

Nice!