You’re not adopting the GPL. Your code is based on WordPress so your code is 100% GPL already.*
So if I am creating a theme for 1 person, that isn’t really used only by myself, does that mean I need to adopt the GPL?
You are using GPL code and your code is GPL as a condition of using GPL’ed code. You can’t accept the license of GPL code and decide that your code from that is not GPL’ed. It is already and you don’t have a choice about that.
That does not mean you need to share that code if you are not distributing it. You do have to follow the GPL if you are sharing (distributing) that code with others.
Example #1: If you wrote a theme for your site, you wrote it, you use it, etc. and you are not giving it to anyone, it is just for your use.
In that scenario you do not have to publish the source. You are not distributing that work. You could share the code but you do not have to.
Here’s a example #2 where you do need to either provide the source or accept that the GPL code you wrote will be shared.
You write a theme. You give it to a site owner. That site owner says “This is the BEST. THEME. IN THE UNIVERSE.” and then proceeds to post the theme with full attribution and credit to you, the creator of that theme.
That would be acceptable for that person to do that. It’s GPL’ed code you shared and you cannot restrict the sharing or modification of that code. That’s the whole point of the GPL.
*Drinks coffee, so good.*
This article may explain it better than I do. ??
https://torquemag.io/2016/11/explaining-and-understanding-the-gnu-general-public-license-gpl/
*The Javascript you include with your theme may not be GPL’ed but almost always is in WordPress themes. Thats because managing different software licenses in a WordPress theme is headache inducing.