Hello!
Emacs 28 recently got built-in support for vertical display in icomplete-mode, distinct from the MELPA package "icomplete-vertical" that Modus already supports. This support comes with a new face, icomplete-selected-match, which by default inherits from highlight.
I find highlight's background a tad too bright; looking into Modus's base faces I find that modus-themes-hl-line yields a more legible result, in addition to making selection in ìcomplete-vertical-mode visually consistent with hl-line-mode.
I don't necessarily think this MR is the best way to solve this problem; I can create an issue instead if more discussion is needed.
At any rate, thank you for your time.