Twenty Seventeen: Table block th has black border in back-end, but not on front-end

[ianbelanger]

While testing the table block vs. the classic block containing a table in the Twenty Seventeen theme, I noticed that a table containing a th will have a black 1px border in the editor, but only the gray bottom-border on the front-end. See screenshot below

[ianbelanger]

Table block with th back-end

[ianbelanger]

Table block with th front-end

[ianbelanger]

Removes the black border from th's in the editor

[ianbelanger]

Showing table block th style is fixed after patch

[mukesh27]

@ianbelanger confirm that 46568.diff​ fix that border issue in Twenty Seventeen.

[ianbelanger]

Thanks @mukesh27, this is all set to commit

[SergeyBiryukov]

In 45051:

Twenty Seventeen: Make <th> styling in table block consistent between back end and front end.

Props ianbelanger.
Fixes #46568.