I'm not sure if eddyg had a different idea, but I think the following sums it up:

  - base16 is a specification of how different UI elements map to colors [1], and also some tooling, configuration files, etc. to automate setting this up for many different applications.
  - For a user to be able to use base16 with a tool, either:
    - Allow the user to set their preferred colors for UI elements at a semantic level, preferrably in a config file, and even more preferrably in a separate file that can be included into the main config file so that it's easier to automate (i.e. I have a separate `colors.sh` that is loaded in my `.zshrc`).
    - Let the user set their shell colors however they want (presumably using [2]), and use the ANSI shell colors (and a few more) according to the base16 style guide [1]. Some translation between shell colors and base16 colors will be necessary, e.g. base16 says that a language keyword should use the color base0E, which in [2] corresponds to ANSI shell color 13.

My impression is that the base16 specification is not sufficiently general for most tools to implement it in a totally unbambiguous way, but if users can set their own colorschemes with a config file, it's not hard for a user to come up with a base16-approximating colorscheme template file, from which other users can generate a config file that sets a specific base16 colorscheme.

[1]: https://github.com/chriskempson/base16/blob/main/styling.md [2]: https://github.com/chriskempson/base16-shell

I'm not sure that I fully understood the theming aspect but one point I think is that the standard 8/8 ANSI colors are presumably picked to be readable by the terminal theme itself, independent from your application.

So if you just use those in your application you don't know the actual colors that the user will see on their screen but you know that they will be something readable.

It’s the first 16 colors of the 256 set. Most current graphical terminals support the latter, but they only expose the first 16 in their configuration. So using them makes your software completely customizable. I believe the tty console only use the first 8.