Ubuntu Theme Switcher Script for Cron (tested on Ubuntu 24.04)

ubuntu_theme_switcher.sh

#!/bin/bash

# Add to `crontab -e`
# */1 * * * * /path/to/ubuntu_theme_switcher.sh
# @reboot /path/to/ubuntu_theme_switcher.sh

# Modified version of https://askubuntu.com/a/1407874
set_theme() {
    if [[ "$1" == "dark" ]]; then
        new_color_scheme="prefer-dark"
    elif [[ "$1" == "light" ]]; then
        new_color_scheme="prefer-light"
    else
        echo "[!] Unsupported theme: $1"
        return
    fi

    # Must be set to work on cron. Get your value running `echo $DBUS_SESSION_BUS_ADDRESS`.
    export DBUS_SESSION_BUS_ADDRESS="unix:path=/run/user/1000/bus"
    
    # Get the current color-scheme setting
    current_color_scheme=$(gsettings get org.gnome.desktop.interface color-scheme)

    # Note the quotes around the value from gsettings
    if [[ "${current_color_scheme}" == "'${new_color_scheme}'" ]]; then
        echo "[i] Already using '${new_color_scheme}' color scheme"
    else
        echo "[-] Setting color scheme to ${new_color_scheme}"
        # Set the new color-scheme
        gsettings set org.gnome.desktop.interface color-scheme ${new_color_scheme}
        # Optionally set theme to Yaru-dark anyways, in my experience things look better this way
        #gsettings set org.gnome.desktop.interface gtk-theme Yaru-dark
        notify-send "[✓] Color scheme changed to ${new_color_scheme}"
    fi
}

# If no argument was provided
if [[ -z "$1" ]]; then
    currenttime=$(date +%H:%M)
    if [[ "$currenttime" > "17:30" || "$currenttime" < "06:00" ]]; then
        set_theme dark
    else
        set_theme light
    fi
else
    set_theme $1
fi