Thank you for writing. What makes the main plot/group window hard to see for you? I know I normally play with the pretty land turned off (I use grey land and the lighter blue water) so that I don't lose information in the hills and valleys. I understand the display can also get pretty small on very high resolution monitors.
With that combination, I have some trouble seeing the light green range circles on the light blue background, and the other lighter colors (white, yellow) are a bit indistinct especially when zoomed out. Switching to dark blue water and black land is easier on my eyes and makes the lighter colors much much clearer, but at the expense of the darker ones (the very important dark blue and the dark red). It's certainly not unplayable, but I find it more difficult than the H3 bright-colors-on-black. I pretty much look at a monitor all day, so my eyes are usually tired.
Another thing about the H3 plot that makes things feel like they're easier to see for me is the ability to use things like datablocks and courses, reference points and zones, and toggling between groups and units. Those things don't make the unit graphics themselves easier to see, but help me to see what's going on at a glance when I make use of them. Particularly with respect to plans re: sub-hunting, coordinated strikes, etc.