Editor: Restores file block button styles for classic themes.

Adds styles into `wp-includes/css/classic-themes.css` to restore file block button element styling for classic themes.

Why?

In [54257] and [54118], button element styles were moved from each block to `theme.json`. For the file block, the download button styles were removed from the block's stylesheet. These changes impacted themes without a `theme.json` file (i.e. classic themes).

This changeset restores the styles for backwards-compatibility.

References:
* [https://github.com/WordPress/gutenberg/pull/47686 Gutenberg PR 47686]

Follow-up to [54257], [54118].

Props wildworks, scruffian, mamaduka, ntsekouras.
Fixes #57688.

git-svn-id: https://develop.svn.wordpress.org/trunk@55323 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/css/classic-themes.css

@@ -16,3 +16,9 @@
 
 	font-size: 1.125em;
 }
+
+.wp-block-file__button {
+	background: #32373c;
+	color: #ffffff;
+	text-decoration: none;
+}