Well, I didn’t got the point of a screenshot tweaking, but okay, lets make a better comparisons.
Same matcaps, defaults with same background (595f75) and vertex size = 4
In 2.93.1 matcaps are a bit darker. Strange, but not a problem, it even should work for contrast better.
But in edit mode the selection is very dim.
Vertices, edges and even faces are hard to detect. There is a huge lack of a contrast, because of wireframe effects processing, that make them bright only at tangent angle (in fresnel style).
Such a wireframe selection requires much more attention to work with.
2.7 Solution is more stable during model rotation.
In wireframe mode we can see that faces in 2.9 also have transparent white shading, while in 2.7 it is transparent black. Of course, it influences contrast in shaded view as well, lowering overall brightness, because is supposed to work with gray background, but not with shaded model.
A gif comparison to feel the effect (indexed, but depicts the difference)
It looks like mesh selection was designed for viewing shape on dense meshes, not for extensive topology editing, especially with image references, which always dims selections even more.