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wp-content/themes/twentytwentyone/assets/sass/05-blocks/file/_editor.scss

@@ -0,0 +1,25 @@
+.wp-block-file {
+
+	.wp-block-file__textlink {
+		text-decoration: underline;
+		text-decoration-style: solid;
+		text-decoration-thickness: 1px;
+
+		&:hover {
+			text-decoration: underline;
+			text-decoration-style: dotted;
+		}
+	}
+
+	.wp-block-file__button {
+		// Extend button style
+		@include button-style();
+		display: inline-block;
+
+		// Remove :focus styles in the editor
+		&:focus {
+			outline-offset: inherit;
+			outline: inherit;
+		}
+	}
+}

wp-content/themes/twentytwentyone/assets/sass/05-blocks/file/_style.scss

@@ -0,0 +1,13 @@
+.wp-block-file {
+
+	// Undo Gutenberg hover defaults
+	a.wp-block-file__button:active,
+	a.wp-block-file__button:focus,
+	a.wp-block-file__button:hover {
+		opacity: inherit;
+	}
+
+	a.wp-block-file__button {
+		display: inline-block;
+	}
+}