So I also wanted to do this but for day/night. I run XFCE and spend a lot of time in neovim. So I started with gitlab.com/bimlas/xfce4-night-mode modified slightly so I could “toggle” or “lock” day/night, setup redshift(-gtk), and wrote a small shell script to query XFCE’s config for which mode is active. XFCE’s settings management is the central point where the info on which theme/mode is active. And then in my .vimrc, I setup a 60s timer to check the day/night mode by shelling out to that query script. Firefox’s Dark Reader extension uses the system color scheme, so even web pages pick up the theme change when it happens.

At the day/night boundaries everything switches color scheme which is nice.

All of this to say that you can probably achieve what you want with some duct tape shell scripts in your /home without needing to rebuild anything. GTK+/Qt both support dynamic theming that can be driven by shell script.