The nature of blur means that more light bleeds into lesser light. Thus light letters on dark backgrounds will bleed the white onto the dark parts. Dark letters on light backgrounds do the same, but the letters still seem dark because they are juxtaposed against the majority-light background. If you switched to your full prescription (for that distance), the dark letters will get much more contrasty and darker. It’s the nature of blur.
However, there’s another factor that occurs, and is the reason we can still see letters despite too-weak glasses. The eye has Spherical Aberration, what that means is that different parts of the cornea/lens focus to different distances. The central part focuses further than the outer part for example.
The way this manifests itself is that when the central part is finally in focus but the rest of the eye lens isn’t, part of the view is sharp (with blur smearing it all). We can see that sharp part. In the example of the light letters on dark background, the letter can appear sharp, but have halos and smears all around it. This means that part of your cornea/lens is finally “there” can can see the letters sharp, but the rest isn’t.
When things get dark (e.g. dusk), our pupils open up. This means that more and more of the light you use to see come from the outer areas of the cornea/lens. Since they focus closer than the center, eyesight appears to deteriorate. But if you look carefully, and have a prescription that (just barely) lets you see sharp lettering in bright sunlight, you will see that those letters are still faintly sharp, just with smearing outside. Again, the nature of our eyes
One of the things that contributes to lens-induced myopia is the fact optometrists tend to prescribe for that low-light (e.g. dusk) situation, thus giving you a prescription for our outer lens/cornea to make that sharp. But that’s too strong (by 0.25-0.5 diopters or more) for the central area that you use in bright daylight. This leads to greater accommodation requirements in good light leading to more myopia progression. But without that overprescribed glasses, you can’t see that well in lower light situations (e.g. driving at dusk/night) that can cause problems (like car accidents).
Thus why I have several “normals” I have higher prescriptions for driving at dusk/night to stay safe, while retaining the blur challenge in good lighting with a (slightly) weaker prescription.