[repo] Meet WCAG AA level for Valid/Invalid example components

The validexample and invalidexample components fail the WCAG AA level
because of the lack of colour contrast of the text.
Changing the success and danger color fixes the issues.

src/css/custom.css

@@ -16,6 +16,8 @@
   --ifm-color-primary-lighter: #d96900;
   --ifm-color-primary-lightest: #f67600;
   --ifm-color-contrasting: #194866;
+  --ifm-color-success: #008a00;
+  --ifm-color-danger: #ed060d;
   --ifm-font-family-base: "Open Sans";
   --ifm-navbar-height: 5.5rem;
   --ifm-navbar-padding-horizontal: 5.5rem;