Use string literals for the TypeScript enum values

enum Theme {
    DEFAULT,
    LIGHT,
    DARK,
}

console.log(Theme.DARK); // 2

enum Theme {
    DEFAULT = 0,
    LIGHT = 1,
    DARK = 2,
}

// TypeScript doesn't throw errors
const theme: Theme.DEFAULT = 3;

enum Theme {
    DEFAULT = 'Default',
    LIGHT = 'Light',
    DARK = 'Dark',
}

console.log(Theme.DARK);                // 'Dark'
let theme: Theme.DEFAULT = 'Default';   // ERROR