Just invoke the :change-window-alpha command on your window. Pass a negative argument to decrease the alpha value (increase transparency), or a positive argument to increase the alpha value (decrease transparency). This command is bound in a transient keymap, enabled by Win + A, in the example config.
To automatically set the alpha value for selected windows:
(:add-hook (in jwno/context :hook-manager) :window-created
(fn [win uia _exe _desktop]
(def class-name (:get_CachedClassName uia))
(when (find |(= $ class-name)
["Emacs"
"ConsoleWindowClass" # Console windows
"CASCADIA_HOSTING_WINDOW_CLASS" # Windows Terminal windows
])
(:set-alpha win (math/floor (* 256 0.9))))))It will set the windows with the specified class names to 90% opaqueness (10% transparency).
Note that not all windows' alpha values can be altered like this.