[FLI-538] feat: add an ENV variable to default theme

[markphelps]

          Would it make sense to add an ENV variable to force a specific theme for everyone?

Originally posted by @gaby in #1933 (comment)

_FLI-538

[markphelps]

This would be a good first issue for anyone interested in implementing

I think it would require the following changes:

1. Add a field here ui.default_theme

   flipt/internal/config/ui.go

   Line 11 in 4a8352a

   Enabled bool `json:"enabled" mapstructure:"enabled"`

   with enum options of system, dark, light
2. Populate the local storage item (

   flipt/ui/src/store.ts

   Line 43 in 4a8352a

   localStorage.getItem(preferencesKey) || '{}'

   ) from this selector https:/flipt-io/flipt/blob/4a8352ae9bb4d854d4e04022ccbba5dd99f4553a/ui/src/app/meta/metaSlice.ts#L51C17-L51C17 for `theme.

The flow would be:

1. Flipt UI reads the ui.default_theme property from the JSON response of the /config endpoint call
2. Flipt UI sets that value in localstorage if it doesn't exist, which then the UI will use as the default theme

[Jamess-Lucass]

I'd be interested in taking this one to get acquainted with the repo.