I am fine with the new monochrome icons that can be drawn in any single color, for most of the interface. Most of the time it is nice to have icons that are not "in your face" and distracting.
However, there are times when higher contrast is important. Most importantly when we use them to highlight an alert, warning, or error.
The following image illustrates this well. The first row shows what alert messages look like currently using the text color and having no impact at all toward alerting the user. The second row shows the same icons but drawn in an alternative color. But using just a single color has limitations, as you can see with the question and cancel icons.
The third row is what they look like with this (experimental) patch. It is drawing one icon with a single color, then another icon on top in a second color. You should that in these cases the extra contrast helps to illustrate the importance of the situation.
The reason this is a "proof of concept" is just to illustrate that drawing two icons on top of one another in two colors is possible and can be of assistance at times.
Personally I would not do this all over the interface. In fact, if we just wanted these three icons to be color we could just leave them in color in the source SVG and exported PNG files.
But where this technique could shine is for the three edit-mode "component selection icons". For these three we could break the icons into two pieces so we could draw the majority of it in TH_TEXT as usual, but then draw the second icon (showing the selected part) in TH_VERTEX_SELECT. This way they would better highlight the selected part, have them look like they did in 2.79, and still be able to change both colors in theme settings.