Get some Oakley replacement lenses. Light grey or VR50 are probably the easiest options.
I've got the the Light Gray (~40% VLT), those are good until sunset. Anything with a light transmission of >35% will be good until about sunset (depending on your low light vision). Higher values will be better until later in the evening. I use the light grey as my everyday riding lens. Its enough tinting for everything but the brightest days, and it works well in shadows, overcast and dusk conditions.