I feel it would be cool to make it to where the watch face would not only have the option to dimmer when you put your watch in standby but if you could, also do it by day and night cycle. So if it’s night the dimmer functionality activates to make it more like a real watch face.
I don’t know whether the tag is working without earlier reported trouble, but there already is a way.
The night design elements should be able to set visible in opacity with use of expression like
Not exactly what you are talking about, but I do that with my personal watch faces. This example face dims the active face from 9pm to 7am. Inspection is open so you can see how I do it. You can also do like @petruuccios says and use the #DISDAYTIME# tags to darken the face from sunset to sunrise. Lots of options here.