Decode JSON string enum into elm union type

A.elm

module A exposing (..)

import Json.Decode as JD
import Json.Encode as JE


type alias User =
    { id : Int
    , theme : Theme
    }


type Theme
    = Light
    | Dark


userDecoder =
    (JD.map2 User
        ( JD.field "id" JD.int )
        ( JD.field "theme" decodeTheme )
    )

-- Can't figure out how to make a custom Json.Decoder.Decoder a
decodeTheme toDecode =
    let
        valueDe =
            JD.decodeString JD.string toDecode
    in
        case valueDe of
            Ok name ->
                if (String.toLower name) == "light" then
                    Light
                else
                    Dark

            Err msg ->
                Dark

Great example!!