I think it is almost impossible to achieve a single theme that fits with both AutoCAD themes (light and dark).
Black works great for my apps.
Also now I dedicate my time to change the size or color of the icons instead of implementing new features!
You're making it more difficult than it needs to be.
Using Gimp, or Photoshop, etc., have a single 'template' for each icon you need, using a resolution of 1,000 x 1,000 pixels, and simply toggle each theme's layer on (the other off) before you export as PNG with Alpha (transparency).
AutoCAD will automagically resize your 1,000 x 1,000 PNG down to whatever icon size it needs for small/large.
If you have 200 icons, and you decide to use different colors for each theme - yes, you'll end up with 400 icons, but that's a heck of a lot better than 800 (200 @ 32x32 light, 200 @ 16x16 light, 200 @ 32x32 dark, 200 @ 16x16 dark)!
Cheers