Index: src/wp-content/themes/twentyfourteen/css/blocks.css
===================================================================
--- src/wp-content/themes/twentyfourteen/css/blocks.css	(revision 55907)
+++ src/wp-content/themes/twentyfourteen/css/blocks.css	(working copy)
@@ -157,6 +157,11 @@
 	background-color: #55d737;
 }
 
+.widget .wp-block-file .wp-block-file__button,
+.widget .wp-block-file .wp-block-file__button:hover {
+	color: #fff;
+}
+
 /*--------------------------------------------------------------
 3.0 Blocks - Formatting
 --------------------------------------------------------------*/
@@ -263,6 +268,11 @@
 	background-color: #55d737;
 }
 
+.widget a.wp-block-button__link:not(.has-text-color),
+.widget a.wp-block-button__link:not(.has-text-color):hover {
+	color: #fff;
+}
+
 /* Separator */
 
 .wp-block-separator {