themes: Fix and improve linked button style
Please note that this is an alternative of !440 (closed), based on !440 (comment 385447).
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)