fix(netlify-cms-ui-default): use grayDark for button (#6069)

* fix(netlify-cms-ui-default): use grayDark for button

This PR is a fix for
#1333 (comment)
because it changes the background color used by `button` to `grayDark`
so that it is accessible.

* chore: update AuthenticationPage test snapshot

Author: jsjoeio

packages/netlify-cms-backend-git-gateway/src/__tests__/__snapshots__/AuthenticationPage.spec.js.snap

@@ -140,7 +140,7 @@ exports[`GitGatewayAuthenticationPage should render with no identity error 1`] =
   padding: 0 15px;
   background-color: #798291;
   color: #fff;
-  background-color: #798291;
+  background-color: #313d3e;
   color: #fff;
   padding: 0 12px;
   margin-top: -40px;
@@ -232,7 +232,7 @@ exports[`GitGatewayAuthenticationPage should render with no identity error 1`] =
   padding: 0 15px;
   background-color: #798291;
   color: #fff;
-  background-color: #798291;
+  background-color: #313d3e;
   color: #fff;
   padding: 0 12px;
   margin-top: -40px;

packages/netlify-cms-ui-default/src/styles.js

@@ -64,7 +64,7 @@ const colors = {
   active: colorsRaw.blue,
   activeBackground: colorsRaw.blueLight,
   inactive: colorsRaw.gray,
-  button: colorsRaw.gray,
+  button: colorsRaw.grayDark,
   buttonText: colorsRaw.white,
   inputBackground: colorsRaw.white,
   infoText: colorsRaw.blue,