From what I can tell, apps, ALL APPS I've used, detect that I have a system light theme. I assume the Zorin Appearance Dark does not in fact tell apps it's a system dark theme, just generates a GTK theme.
Is there a terminal command or app I can install to make my system have a dark theme
I am having a lot of difficulty understanding this.
You are correct that Zorin Appearance does not "detect" the dark or light theme, it sets the theme you choose. You can use the Zorin Appearance App to use a Dark theme, which is what you ask in the second question:
You also can use gnome-tweaks on Zorin OS Core to set the theme.
Are you asking whether you can have the system detect whether you are using a Dark or Light theme in order to automatically switch between the two depending on time of day?
Could you specify what do you mean by 'detect'?? Because if you already set your System to use Dark Theme in the Zorin Appearance, all apps that preinstalled and you installed by yourself are automatically using Dark Theme(as long as they are GTK-based Apps).
As far as I know, I don't think websites can automatically follow your system theme, for example Youtube. It can't automatically follow the Dark Theme that you've set up for your system, but you can set YT itself to use Dark Mode.
Zorin has and announces that it is using either the light or dark theme. It is up to the application developers to integrate detection and theme support, not the Zorin developers (they did their part). The themes and the OS were tested extensively, both by the developers and users of this forum. Many bugs or inconsistencies were detected and resolved before the release of 16. Others have been addressed since (based mostly on hardware specificity and combinations).
If there is a program that you can specify, we may be able to determine if that developer has included theme detection or just offered theme choice, if any.
The OS is not responsible for other software's theme content or detection. Even android depends on the app developers to include such features. Linked in only recently provided support for such theme options, while some bank apps still require you to endure glaring white (as well as many underdeveloped applications).
I have accurately answered your question in response to your own examples and in Windows.
Windows uses the Color Filter Application in order to achieve the Light or Dark mode options even in Websites.
This is handled by the Display. Not by changing the website HTML in any way.
In order to address how you can achieve the results you want, all members of this topic must be able to understand how it works and what can be done to duplicate the results.
Which I cannot achieve if you reject or dismiss the answers by arguing with each and every one every step of the way. It will not change whether you believe me or not; Windows uses a color filtration in order to achieve that affect.
As @337harvey pointed out, Zorin OS includes an application to auto theme. This can switch your theme from Light Or Dark depending on the times you set it to switch.
This is Not Windows and it does not use Windows Desktop Environment. As such, it does not have Windows Color Filter that can invert contrasts.
To this end, I recommend Dark Reader or another good extension for use in browsers for webpages.
If you are experiencing troubles with the Zorin Application for autotheme not working, then please clarify that and what steps you have already taken to troubleshoot.