Dusk to dawn dimmable control for indoor stair lights
The light sensor is obsolete

Modern light sensors "learn".  It takes them a day or two to learn "This is as bright as it gets" and "this is as dark as it gets", but once it does so, it will correctly switch the dusk-to-dawn lighting.  However it loses the memory if you switch power off to it.  So don't do that.

So toss your obsolete light sensor and get a new one. 

One does not simply "dim" LEDs

Your best bet is simply get smaller (lower lumen) LED "bulbs" if it takes bulbs. 

Your second best bet is get some cinematic gels (a sheet of tinted plastic).  Figure a way to tuck them inside your fixtures and cut them to shape. If it's not enough dimming, use two or more.  I'd use light gray, and also it's a good time to correct color temperature issues, that's the main use of gels. 

Third best is to get an IoT/smart home/connected home controller, and get LED lights built to be dimmable by that smart controller.  The smart controller has no sensor; it gets on the Internet and finds out when dusk and dawn are. Then you implement both the dimming and the dusk-to-dawn with controller scripts, "at dawn turn off lights 1,2,3,4,5" "at dusk set lights 1,2,3,4,5 to 27%". 

Other than that, dimming LEDs is byzantine.  Most of them have a switching power supply designed to work on multiple voltages, and will see dimming as a "brownout" and either refuse to dim, or shut off.  You have to buy particular LEDs designed to be dimmed, note which particular technology of dimmer they work with, and get a compatible dimmer. But most of them assume the human wants to change the dimming daily, they are not made to set once and hold forever.  After a power failure they'll likely bounce back to full brightness.  Generally unless you've designed the system to be dimmable, dimming ain't gonna happen.

And mind you, all that stuff in the last paragraph probably won't "play well" with the dusk-to-dawn sensor.  Either the dimmer will "dim" the sensor which will cause problems, or the sensor will shut off power to the dimmer and reset it.

You're asking a lot of technology here and you'll have to cope with some compromises.