Is this the contrast on my screen, or others have the same issue? Selection boxes are not visible. People who don’t know that they are there will probably miss them. (also this is from a 4K screen. I think the gray edge is too thin. I can actually see the faint trace of it the attached screen capture, but not during in the application).
It appears in light mode that border of the checkbox is #c8c8c8 and based on the values specified for Dark Mode, this color appears to be defined by QPalette::Mid (see here). Oddly in the Slicer light style these are the same value when they shouldn’t be. Read about QPalette ColorRoles.
It is harder to see in dark mode because QPalette::Mid is closer to QPalette::Base(the filled color), than it is in light mode. This is a weird case of a widget (QCheckbox) that is using the color of QPalette::Base even though it is within another widget (QTableWidget) which also uses QPalette::Base by default for its contents. Setting it to use the QPalette::AlternateBase could potentially work assuming that this widget continues to not use alternatingRowColors.