Clojurians Log v2

consult-theme is also handy

...to test things out.

it has live previews so you can super quickly browse tons of themes and compare them

speaking of faces, there's also https://github.com/Fanael/highlight-defined package, which can highlight faces (among other emacs lisp things) based on their names, quite useful for creating themes. Here's my config:

(use-package highlight-defined
  :ensure t
  :custom
  (highlight-defined-face-use-itself t) ; <- this
  :hook
  (help-mode . highlight-defined-mode)
  (emacs-lisp-mode . highlight-defined-mode))

That looks handy! Never heard of it, but one can never keep track of all the great Emacs packages out there!

So, when designing themes, I usually create a quick macro (F3; M-x disable-theme; M-x load-theme; F4) and I can see all the changes perfectly in a fraction of a second without having to open a file with a complex syntax.