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>

docs/sources/setup-grafana/configure-grafana/_index.md

@@ -772,7 +772,9 @@ Text used as placeholder text on login page for password input.
 
 ### default_theme
 
-Set the default UI theme: `dark` or `light`. Default is `dark`.
+Sets the default UI theme: `dark`, `light`, or `system`. The default theme is `dark`.
+
+`system` matches the user's system theme.
 
 ### default_language