I prefer Marwaita icons and think those, plus the Adapta theme should be Linux Mint's default themes, but then again, that's most likely because I personally love those themes, probably not everyone feels the same way about them.
I personally really don't like dark themes either (most other people seem to really love dark themes thou) I just don't see the charm.. However, Adapta-Notko (Adapta's dark theme) is the most highest rated downloadable theme in the theme-settings. I however prefer the lighter version, a good mix of dark and light colours.
The reason I like the Marwaita icons are because they feel pretty streamlined, look very professional and serious. I'm not a fan of the square design of Mint's default icons, I used to like that design back when I was still rocking Ubuntu with Unity, but ever since moving to Linux Mint with Cinnamon, I just had to change, the square icons does not look good as desktop icons (which Cinnamon enables by default.) ..In my view.
