Re-format existing card files and add plain color cards.

Signed-off-by: Kouya Heika <kouyaheika@canithesis.org>

card/plain/black.json

@@ -0,0 +1,8 @@
+{
+	"black": {
+		"": {
+			"--usercard-color": "#000000",
+			"--main-text": "#f6f5f4"
+		}
+	}
+}