[Prot Edit: Use `modus-themes-hl-line`] Make selections accessible as well

Modus Themes pride themselves on being accessible, and I applaud the goal.

However, as of now, one of the less accessible aspects of Modus Themes is selections. Below is a screenshot from mu4e with a selected message, and I can barely discern it, especially when sitting further from the computer. That said, the problem is not about mu4e specifically. I have the same problem in Magit, where I often get lost, unable to discern which file and the hunk is currently selected.

It is worth noting that, in the case of selections, the foreground-to-background contrast ratio effectively becomes the background-to-background contrast ratio, as the selection is the content "on top" of its background. And because selections are some of the most important pieces of information, they should follow the highest standard for color contrast as well.