'Soft Light Closure' and 'fully dark closure'.
Defining Terms to make your document easier and clearer.
I would give the state of 'light peeking in' a name of its own. to make it easier to talk about it.
I mean, I'd give it a product name like 'Soft Light' or something like that. Then, you can easily talk about 'Soft Light' closure. And you can also then talk about 'fully dark closure' without tying yourself up in knots.
You'll then be able to refer to the 'time for Soft Light closure' and the 'time for fully dark closure' also, without tying yourself up in knots.
You'd start off your document with something like this:
There's a special way of closing our blinds so that light still still peeks in - the shutter is fully secure and closed but some light comes in through unique small apertures. We call this feature 'Soft Light', and we refer to 'Soft Light Closure' when your shutters are like this. When your shutters are fully closed, and completely dark (ie, no apertures are open) we call it 'fully dark closure'.
Having stated this once at the start, you can just refer to 'Soft Light closure' and 'fully dark closure' continually, after that.
Often, legal documents use this technique - they define certain terms up front - and then refer to them in exactly the same way throughout the entire document. This method is very simple and clear, and I feel it is also well used in brochures or technical documents of the kind that you seem to be making.
You refer to your 'terms' up front, in the introduction - and then refer to them throughout. They can even be listed as 'terms used throughout this document'
As you do seem to have a unique 'product feature' here - why not give it a name?
Note: 'Soft Light' has capitals as it is a suggested 'product name'. 'Fully dark closure' does not, as it is a normal feature of shutters, not a product name.
Here's a good, more detailed description about how the concept of defining terms, described here, is used in legal documents.
https://www.netlawman.co.uk/ia/defined-terms-in-legal-documents