modals: Fix modal buttons in Safari for some users.
modals: Fix modal buttons in Safari for some users.

Some Safari users users observed an issue where these modal buttons
appeared with an ugly white-on-grey format. Removing the CSS line with
`appearance: button` appears to fix this for affected users.

That line seems unlikely to have been constructive; as documented in
https://developer.mozilla.org/en-US/docs/Web/CSS/appearance, this is a
deprecated value equivalent to `auto`, and a bit of a misfeature.

Additionally, the "Log in" button was incorrectly underlined on hover;
override that style.

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