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
You can go to zorin appearance in the start menu, this will surely provide you a dark theme. Then you might select some dark wallpapers to truly make your PC give the Dark Feeling 
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?
Nope. Apps don't know. I want software to pick up my system theme as dark. I can change GTK theme all I want.
You misunderstand. Apps and website detect my system theme as light and not dark.
I want my system to be detected as dark by websites, apps, etc. (NOT THE GTK THEME)
Maybe, A driver issue, you can go to your browser setting and select dark theme there. This maybe because the setting maybe set to light by default not to detect system theme or dark.
Apps should follow the GTK Theme. Your system theme is set by the GTK theme, and whether it is light or dark is also set by the GTK theme.
It may help if you outline whether some apps show a light theme and which apps sepcifically.
Websites do not follow the GTK Theme, but the HTML of the site.
You can use browser extensions for giving all websites a dark theme. I sue Dark Reader.
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).
There is often a "system" theme option along with light and dark on websites.
The "system theme" option is always light.
They can, it works on Windows when i used to use that. System theme is dark? All sites pick up it's dark and are dark.
Windows uses a Color Filter to invert colors displayed on your screen.
The page HTML stays the same.
Windows also does not utilize the GTK3 that we use.
So far, I am not aware of any System-wide Color filter on Linux.
This is why I use Dark Reader extension for Webpages.
And I use Dark Themes for GTK.
Many browsers on Linux also have a setting for using either the in-built browser appearance or for using the system GTK theme - which should be checked by the user.
Nope. I don't mean any filter. I mean this:
The "auto" or "system" option.
These are based on what mode you set your system to when in windows.
@kzhe ...
That is the color filter app that I was referring to.
The setting you show is what sets the color filter to invert.
No, it's not a filter, i change it in the native website and they all do their own thing.
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.
Sorry, I got confused by language and thought- yeah sorry about that, and thanks for the help.
Wait... I remember being able to run Vivaldi with a command and having it detect as dark... ?
nevermind that was a vivaldi internal thing/ chromium thing with the --force-dark-mode flag