themes: Fix and improve linked button style

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
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