Changing appearance of core applications while using i3 window manager

[skyeye...@gmail.com]

I switched over to i3 recently and have been browsing this link for changing GUI appearance in some apps:

https://www.qubes-os.org/doc/dark-theme/

I've also tried the following:

gnome-tweak-tool
lxappearance

Despite those tools, I can't seem to change the appearance of default core applications like Qubes Manager, Files, Calendar keep etc. Any suggestions on how to do that?