fix: make sure custom colours don't override newsletter modern style (#…

…2427)

newspack-theme/sass/blocks/_blocks.scss

@@ -359,6 +359,17 @@ p.has-background {
 	}
 }
 
+//! Newspack Newlsetter Subscription Block
+.wp-block-newspack-newsletters-subscribe {
+	// Make sure custom colors don't override the color that should be used for the modern style.
+	&.is-style-modern input[type="checkbox"]::before {
+		background:
+			transparent
+			url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 24 24' width='24' height='24'%3E%3Cpath d='M16.7 7.1l-6.3 8.5-3.3-2.5-.9 1.2 4.5 3.4L17.9 8z' fill='white' %3E%3C/path%3E%3C/svg%3E")
+			0 0 no-repeat;
+	}
+}
+
 //! Columns
 .wp-block-columns {
 	.wp-block-cover,