A shell script to toggle between light and dark variants of a GTK theme.

dark-toggle.sh

#!/bin/sh

# A small POSIX compliant script to toggle between dark and light variant
# of a theme for GNOME based desktops.

# Copyright (C) 2021 Rifaz Nahiyan
# This code is licensed under the MIT License.
# View the license in its entirety at: https://opensource.org/licenses/MIT

get_current_theme () {
    gsettings get org.gnome.desktop.interface gtk-theme | tr --delete \'
}

set_theme () {
    gsettings set org.gnome.desktop.interface gtk-theme "$1"
    # Unfortunately, gsettings always reports exit status 0
}

## Sanity checks
deps_check () {
	deps="gsettings notify-send"
	missing=""
	for dep in ${deps}; do
		command -v "${dep}" >/dev/null || missing="${missing} ${dep}"
	done
	if [ -n "${missing}" ]
	then
		die "Missing necessary dependencies: ${missing}"
	fi
	unset missing
}

# Display a formatted message, then exit with error
die () {
	printf "%s: %s\n" "${0##*/}" "${*}" >&2
	exit 1
}

# Get the script's basename and trim the '.sh' (if any) at the end
SCRIPTNAME="${0##*/}"
SCRIPTNAME="${SCRIPTNAME%.sh}"

main () {
    # Check if necessary and optional dependecies are there, else exit
    deps_check

    current_theme="$(get_current_theme)"

    # Check if the theme name has "dark" or "Dark" at the end of its name,
    # then set new theme accordingly
    case $current_theme in
        *-[dD]ark)
            new_theme="${current_theme%-[Dd]ark}"
            ;;
        *)
            # Extra check for Arc theme as it breaks convention by using captial 'D'
            if [ "$current_theme" = "Arc" ]; then
                new_theme="$current_theme"-Dark
            else
                new_theme="$current_theme"-dark
            fi
            DARK="dark"
            ;;
    esac

    set_theme "$new_theme"
    notify_msg="Theme switched to ${DARK:-light} variant."
    notify-send -t 5000 "${SCRIPTNAME}" "$notify_msg"
}

main "${@}"

Right. Exiting with 1 almost objectively makes more sense and we are also printing to stderr for removing ambiguity. Also, command -v $program itself exits with 1 and not 127 or something, so we can piggy-back off that.

About checking whether a theme exists, gsettings does not provide any API for that, so the best thing currently to do is to check whether a directory with the theme name exists in: /usr/share/themes/ and ~/.themes/.

Updated code with the dependency checks.