How can I get my SVG icon to automatically change its colour depending on the background colour?

I am trying to add some icons into ~/.local/share/icons/ so that I can use them with notify-send. I can do that just fine, but the icon colour is not adapting to the background, unlike the Adwaita icons, and the contrast ends up being incredibly low. Note that I am able to just change the colour of the icon, but this is not a viable solution, as the icon is meant to be shared with other people, who may have light themes for their notifications. Here is what an Adwaita icon looks like in a GNOME notification: And here is what my other icon looks like in a GNOME notification: Both .svg files have the same base colour (#2e3436), but only the Adwaita one changes automatically to be legible even on a dark background. I converted my .svg file and edited it so that it resembles the Adwaita one, but I still have the same issue. Here is what the Adwaita icon code looks like:

And here is what I made the other icon into: