Themes: Adds support for extraThemes (behind feature toggle) (#67981)

* Foundations to support more themes

* Fixes

* add another test theme

* Refactorings

* more refactoring

* Update

* Fixing tests

* Fix

* Update

pkg/services/preference/themes.go

@@ -0,0 +1,34 @@
+package pref
+
+type ThemeDTO struct {
+	ID      string `json:"id"`
+	Type    string `json:"type"`
+	IsExtra bool   `json:"isExtra"`
+}
+
+var themes = []ThemeDTO{
+	{ID: "light", Type: "light"},
+	{ID: "dark", Type: "dark"},
+	{ID: "system", Type: "dark"},
+	{ID: "midnight", Type: "dark", IsExtra: true},
+	{ID: "blue-night", Type: "dark", IsExtra: true},
+}
+
+func GetThemeByID(id string) *ThemeDTO {
+	for _, theme := range themes {
+		if theme.ID == id {
+			return &theme
+		}
+	}
+
+	return nil
+}
+
+func IsValidThemeID(id string) bool {
+	for _, theme := range themes {
+		if theme.ID == id {
+			return true
+		}
+	}
+	return false
+}