Add theme documentation.

themes/README.md

@@ -1 +1,15 @@
-TODO: document custom themes here...
+# Custom Fairspace themes for Keycloak
+
+Keycloak allows customising the look and feel of the web pages, including login, account or admin console, by providing custom themes.
+Keycloak provides default themes that can be extended or overridden by custom themes. A theme consists of a set of resources, such as HTML templates, CSS, JavaScript, images, message bundles and theme properties.
+
+More information about custom themes can be found in the [Keycloak documentation](https://www.keycloak.org/docs/latest/server_development/#_theme_stylesheet).
+
+
+## Fairspace theme
+
+This directory contains the custom Fairspace theme for Keycloak. The theme is based on the default theme called "keycloak" and includes custom styles and a logo.
+
+The theme is packaged into a Docker image and made available in the Kubernetes setup using init container as theme provider. 
+
+More information about the theme setup with Kubernetes and Helm can be found in [Keycloakx Helm chart documentation](https://github.com/codecentric/helm-charts/tree/master/charts/keycloakx#providing-a-custom-theme).