CHANGE: style ID of the switched style is set in the style properties
Now you can have more Light styles and set the switched style (i.e. Dark style) for any of the Light ones.
To see it live as a demo look at SynoForum.com - The Unofficial Synology Forum. You can switch between Light and Dark style AND VertiLight and VertiDark style. To choose VertiLight style, switch the style from the footer, and then you can use the switch for VertiDark.
NOTE: if you are upgrading from the previous version, make sure to check the Style properties and set the Style ID