themes: Fix and improve linked button style

Please note that this is an alternative of !440 (closed), based on !440 (comment 385447).

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Instead of re-styling the border and radius of the linked buttons depending on the position (middle, first, last, or only child), just remove the border and radius in a specific direction when the button is :not(:first-child) and/or :not(:last-child).

This properly handles the style of linked buttons in all positions -- middle, first, last, and only child.

Fixes #1294 (closed)