Preferences: Add theme preference to match system theme (#61986)

* user essentials mob! 🔱

lastFile:pkg/api/preferences.go

* user essentials mob! 🔱

* user essentials mob! 🔱

lastFile:packages/grafana-data/src/types/config.ts

* user essentials mob! 🔱

lastFile:public/app/core/services/echo/utils.test.ts

* user essentials mob! 🔱

* user essentials mob! 🔱

lastFile:public/views/index-template.html

* user essentials mob! 🔱

* Restore currentUser.lightTheme for backwards compat

* fix types

* Apply suggestions from code review

Co-authored-by: Christopher Moyer <35463610+chri2547@users.noreply.github.com>

* cleanup

* cleanup

---------

Co-authored-by: Ashley Harrison <ashley.harrison@grafana.com>
Co-authored-by: Joao Silva <joao.silva@grafana.com>
Co-authored-by: Christopher Moyer <35463610+chri2547@users.noreply.github.com>

pkg/api/dtos/models.go

@@ -33,7 +33,8 @@ type CurrentUser struct {
 	Login                      string             `json:"login"`
 	Email                      string             `json:"email"`
 	Name                       string             `json:"name"`
-	LightTheme                 bool               `json:"lightTheme"`
+	Theme                      string             `json:"theme"`
+	LightTheme                 bool               `json:"lightTheme"` // deprecated, use theme instead
 	OrgCount                   int                `json:"orgCount"`
 	OrgId                      int64              `json:"orgId"`
 	OrgName                    string             `json:"orgName"`