Nice catch, I didn't even realize that the taskbar wasn't changing. I've wrote a new version that fixes this and also makes sure that the background changes accordingly, something that I also missed before.
Remember that this is still a rudimentary script and acts only as a work around, but if you find any other issues let me know and I'll try to fix them.
This is the new script, all you have to do is replace the contents from the file you already have with this:
#!/bin/bash
accent_color=Blue
theme_color=Light
color_scheme='prefer-light'
start_hour=18
end_hour=6
current_hour=$(date +%H)
if [[ $current_hour -ge $start_hour || $current_hour -le $end_hour ]]
then
theme_color=Dark
color_scheme=prefer-dark
fi
theme_str="Zorin${accent_color}-${theme_color}"
gsettings set org.gnome.desktop.interface gtk-theme "$theme_str"
gsettings set org.gnome.desktop.interface color-scheme "$color_scheme"
gsettings set org.gnome.shell.extensions.user-theme name "$theme_str"
I'm using Flameshot for the arrows and markings. It's quite a handy program for this type of screenshots where things need to be pointed out.