chore(theming): remove Sass default flag & update guidelines (#29458)

Remove a missed `!default` flag in the themes directory and update the
Sass variable guidelines docs to eliminate the `!default` flag from the
code examples, except for in the historic usage section which includes
examples from previous versions where the `!default` flag was used.

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Co-authored-by: Brandy Carney <brandy@ionic.io>