I like both, depending on how much light is in the room. I much prefer to set my desktop light or dark manually than to automate with timers.

On GNOME light or dark is color-scheme and you can set read and set it from the command line. You can read it from the command line (default light theme is called default)

$ gsettings get org.gnome.desktop.interface color-scheme | tr -d "'"
default

and you can set it from the command line (default dark theme is called prefer-dark)

$ gsettings set org.gnome.desktop.interface color-scheme prefer-dark

will change it to dark.

(You can also set other preferred GNOME themes this way.)

Protesilaos has published a gazillion pretty themes, but I prefer his modus-themes for their visual simplicity and also for their UI amenities. For example you can set one light theme (modus-operandi family) and one dark theme (modus-vivendi family) and toggle between them:

(setopt modus-themes-to-toggle '(modus-operandi-tinted modus-vivendi-tinted))
(bind-key "C-c m" #'modus-themes-toggle)

And there is the custom-variable custom-enabled-themes, usually set in your custom-file, that determines what themes are initially enabled.

(setopt custom-enabled-themes '(modus-operandi-tinted modus-vivendi-tinted))

The order of this list determines which theme is activated by default: the first one.

But when I start Emacs, I want the startup theme to match the GNOME theme! So I took custom-enabled-themes out of the custom file and added these lines to early-init.el:

(defconst jeh/modus-themes-list
  (let* ((light 'modus-operandi-tinted)
         (dark 'modus-vivendi-tinted)
         (gnome-theme
          (car (split-string
                (shell-command-to-string
                 "gsettings get org.gnome.desktop.interface color-scheme | tr -d \"'\"")))))
    (cond ((string= gnome-theme "default")
           `(,light ,dark))
          ((string= gnome-theme "prefer-dark")
           `(,dark ,light))))
  "List of modus themes IN ORDER: first element is the default theme,
which depends on whether the GNOME color-scheme is light or dark.")

(setopt custom-enabled-themes  jeh/modus-themes-list)
(setopt modus-themes-to-toggle jeh/modus-themes-list)

I wish I could remember from whom I stole this little bash script to toggle GNOME theme. Note the last line I added, which also toggles Emacs theme.

#!/usr/bin/env bash
class=org.gnome.desktop.interface 
name=color-scheme
status=$(gsettings get "$class" "$name" | tr -d "'")
if [[ $status = "default" || $status = "prefer-light" ]]; then
    new_status="prefer-dark"
else
    new_status="default"
fi
gsettings set "$class" "$name" "$new_status"
emacsclient -e '(modus-themes-toggle)'

Save it to somewhere in your path (e.g. ~/bin/gnome-toggle-light-dark.sh) and chmod +x ~/bin/gnome-toggle-light-dark.sh. Test it from the command line.

The best part is that GNOME lets you bind a keyboard shortcut to a script:

Settings ⟶
    Keyboard ⟶
        View and Customize Shortcuts ⟶
            Custom Shortcuts ⟶
                + (add shortcut)

Now you can toggle GNOME and Emacs themes together as the sky determines your mood.