I’m hoping you can experiment a bit more though with the aim of removing the need for theming.
The size and thickness of things should be changing with user scale and DPI. So if it is not, fixing that can address your first point. The vertex dots sizes are already user themed, so if these knife target dot sizes are not related to those that can also be fixed. As for the red-green issue, I personally like your use of white in your mockup better.
Are you on Mac? I am only asking because we have an issue on many of those where they don’t respect a line width greater than a pixel, so line can get harder to see if you use a high-dpi display like Retina.
So mostly just trying to start with a solution that gets us most of what you want but without needing more theme settings.
Hey, hoping that my previous comment didn’t come off as discouraging. It really was the opposite. I was excited to see someone else thinking about this stuff.
Looking into it a bit further, there are lots of areas that don’t scale correctly with UI scale, DPI changes, and line width. And the lines will look extra thin on some Macs because they might see 1 pixel-wide lines that are meant to be two pixels wide.
The colors though, are in the theme, just really hard to find but all in Themes / 3D View. The line you see drawn are “NURBS V-lines”. The red vertices are “Active spline”, while the green ones are “Vector handle selected color”. Yes, all from other things, but still changeable.