Document how to add custom themes (#460)

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### Description of the change

Add documentation how to add custom themes

### Benefits

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### Possible drawbacks

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### Applicable issues

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  - fixes #301

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### Checklist

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- [x] Parameters are documented in the `values.yaml` and added to the `README.md` using [readme-generator-for-helm](https://github.com/bitnami-labs/readme-generator-for-helm)
- [x] Breaking changes are documented in the `README.md`
- [x] Templating unittests are added

Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/460
Co-authored-by: pat-s <patrick.schratz@gmail.com>
Co-committed-by: pat-s <patrick.schratz@gmail.com>

README.md

@@ -21,6 +21,7 @@
 - [Configure commit signing](#configure-commit-signing)
 - [Metrics and profiling](#metrics-and-profiling)
 - [Pod annotations](#pod-annotations)
+- [Themes](#themes)
 - [Parameters](#parameters)
   - [Global](#global)
   - [Image](#image)
@@ -606,6 +607,48 @@ gitea:
   podAnnotations: {}
 ```
 
+## Themes
+
+Custom themes can be added via k8s secrets and referencing them in `values.yaml`.
+
+```yaml
+extraVolumes:
+  - name: gitea-themes
+    secret:
+      secretName: gitea-themes
+
+extraVolumeMounts:
+  - name: gitea-themes
+    readOnly: true
+    mountPath: "/data/gitea/public/css"
+```
+
+The secret can be created via `terraform`:
+
+```hcl
+resource "kubernetes_secret" "gitea-themes" {
+  metadata {
+    name      = "gitea-themes"
+    namespace = "gitea"
+  }
+
+  data = {
+    "theme-custom.css"      = "${file("FULL-PATH-TO-CSS")}"
+    "theme-custom-dark.css" = "${file("FULL-PATH-TO-CSS")}"
+  }
+
+  type = "Opaque"
+
+  depends_on = [kubernetes_namespace.gitea]
+}
+```
+
+or natively via `kubectl`:
+
+```bash
+kubectl create secret generic gitea-themes --from-file={{FULL-PATH-TO-CSS}} --namespace gitea
+```
+
 ## Parameters
 
 ### Global