Thunderbird 115 Supernova uses a really simple method of creating themes.  You basically put tags and color codes into a json file and zip it up. 

Now, I know how to edit the code and set the colours and I know how to make my own custom themes.
However, my question is:
Is there a way to change the colours "live" so you can actually see what they look like?
So far the only thing I can do is guess what it will look like, edit the code, zip the extension and install it and then enable it.  Blech.

I look at the Browser toolbox, but can't find any way to locate the actual elements that I can actually style.
Here's is a json manifest sample with the simple code: https://developer.thunderbird.net/add-ons/web-extension-themes
Maybe you can actually do what I want with the Browser Toolbox, but for the life of me I can't figure it out.

I've googled the world and can't find any answers.
Waaaah! I just wanna adjust the code in the interface and see what it looks like!  Is there no way to preview what you are doing?

So, I come here, hoping somebody might be listening.

Thanks, Shades for answering.  I was trying to get around buying anything.  I realize I could stick a switch or router between that computer and the network, but that was my last resort.  I was hoping more for some super-admin program I could install on the machine.

My query was more for known free software.

Here's a simple solution:
https://crystalrich.com/internetoff/

It's not quite what I had in mind.  This one is meant for the person who wants to disable their own internet - not for the person who wants to disable someone else's, such as a family internet or parental control.  All it does is disable the network adapter.

Anyone?