There seems to be no way to do this. Instead, this is what I went. I added the following to the vimrc file:
autocmd FileType markdown set formatoptions=ant autocmd FileType markdown set textwidth=72 autocmd FileType text set formatoptions-=t
Thus, with markdown and markdown only, each line of the screen is a true line, but they are automatically adjusted.
For the highlight itself, I found the following useful for Solarized dark colorscheme:
hi CursorLine guifg=White guibg=
Instead of a lighter background color for the cursor line, it gives a lighter foreground color. In other words, higher contrast, not lower. It is subtle but effective.
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