[css-ui] UA stylesheet for appearance:base `<select>`

[mozilla-apprentice]

Resolutions were made for csswg-drafts/#10857.

[css-ui] UA stylesheet for appearance:base `<select>`

• RESOLVED: have the same border radius for base appearance for selects and buttons, conditional on giving others not present a week to review.
• RESOLVED: use transparent background color for base appearance selects, buttons, and inputs

Discussion.

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