Although the electrical tape suggestion is a great one, depending on exactly where the light strip is located you could have a problem with mouse movement if there's any contact between it and the surface on which the mouse moves (and it could also take a thin strip off, too).
If that's the case consider painting over the light strip with a couple of coats of black nail enamel or model enamel, either gloss or matte, or even some other color, as in sufficient thickness it will block, or at least very significantly dim, that light.
Tape is way easier if it works, though.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1803, Build 17134
Memory is a crazy woman that hoards rags and throws away food.
~ Austin O'Malley