Style sphinx-design dropdowns closer to admonitions by default

[chrisjsewell]

See sphinx-design v0.6.0 changelog: https://sphinx-design.readthedocs.io/en/furo-theme/changelog.html#improve-dropdown-title-bar

the dropdowns here already apply these variables: https://sphinx-design.readthedocs.io/en/furo-theme/dropdowns.html#dropdowns-in-other-components

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want to also allow for the same colors and left border, but didn't get round to figuring that out yet