Adds the admin-scheme styles as a dependency for the login and install styles. This is to ensure the CSS variables are available to the login, installation, database repair and upgrade screens.
Modifies the display of notices in the login styles to match those in the new default scheme, "modern".
Props peterwilsoncc, wildworks, westonruter, mukesh27, fabiankaegy, audrasjb, huzaifaalmesbah, sabernhardt, presskopp, SirLouen, ellatrix, nendeb55, neo2k23, jsmansart, joedolson.
Fixes#64640, #64548. See #64308.
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