Support color customization from the admin (#2338)

* Add user-customizable theming. Closes #1915

* Prettified Code!

* Add user-customizable theming. Closes #1915

* Add explicit color for page content background

* Prettified Code!

Co-authored-by: gabek <gabek@users.noreply.github.com>

controllers/admin/appearance.go

@@ -0,0 +1,35 @@
+package admin
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/owncast/owncast/controllers"
+	"github.com/owncast/owncast/core/data"
+)
+
+// SetCustomColorVariableValues sets the custom color variables.
+func SetCustomColorVariableValues(w http.ResponseWriter, r *http.Request) {
+	if !requirePOST(w, r) {
+		return
+	}
+
+	type request struct {
+		Value map[string]string `json:"value"`
+	}
+
+	decoder := json.NewDecoder(r.Body)
+	var values request
+
+	if err := decoder.Decode(&values); err != nil {
+		controllers.WriteSimpleResponse(w, false, "unable to update appearance variable values")
+		return
+	}
+
+	if err := data.SetCustomColorVariableValues(values.Value); err != nil {
+		controllers.WriteSimpleResponse(w, false, err.Error())
+		return
+	}
+
+	controllers.WriteSimpleResponse(w, true, "custom appearance variables updated")
+}