Speaking of editing xml, this is what I've done for a simplistic "dark mode" mod here: https://github.com/vgaming/0ad-dark-mode
The mod isn't really "complete" in any sense, but it works for changing all terrains to a dark one. I'd be happy to do more stuff like visually smaller trees in the future, but for now I've time-limited the effort. Will possibly return to it in a few months (I play open-source games in waves, few months playing, few months not).
Thanks for AbstractGUI's mod and videos by the way, it partially inspired me to work on that other add-on, because I've liked how a dark mode *can* look like