Affinity: Update block caption colour; see #459.

affinity/blocks.css

@@ -21,6 +21,7 @@ Description: Used to style Gutenberg Blocks.
 /* Captions */
 
 [class^="wp-block-"]:not(.wp-block-gallery) figcaption {
+	color: inherit;
 	font-size: 13.2px;
 	font-style: italic;
 	margin-bottom: 1.6em;