See the paren-face package. The diagram you mention sounds like the one in the repository README. (I think the diagram has been used in longer article too, but I’m not sure where it originated.)

For a complete list, see the documentation of the marginalia--symbol-class function.

Aren’t those XDA profile keycaps?

I love how “ESC” and “CTRL” are quoted.

We’re expecting RET to perform minibuffer-exit. That’s the default keybinding from minibuffer-mode-map. Perhaps another keybinding (from another minor-mode keymap, say) is overriding the minibuffer’s RET binding.

it just goes to the next line

Hmmm, this sounds familiar. I had this problem last year.

Do you have Paredit mode enabled in the minibuffer? (You might look at the value of eval-expression-minibuffer-setup-hook.)

The paredit-mode-map has the paredit-RET command bound to RET instead. This command usually puts a new line in place.

For variables you created (i.e. named) yourself, you can also use defvar, which lets you give it a docstring.

To set a different value according to the OS, you can conditionally set the value based on a system-type.

    (defvar fuzzbomb/startup-theme 'modus-vivendi-tinted
        "The theme to enable during startup.

    This variable should be set before the first call to `load-theme' in
`init.el'.")

      (if (eq system-type 'windows-nt)
          (setq fuzzbomb/startup-theme 'ef-maris-light))