ThemeToggler adds two buttons under the edit button on the left toolbar to toggle between the light and dark themes on FandomDesktop. The eye icon will toggle the theme and save the theme to your preferences, whereas the clock icon will only temporarily change the theme on the current page. The page does not refresh upon switching between themes.
Note: This script may be unnecessary if you are just looking for a simple toggle between light and dark, as Fandom has added a built-in switcher to FandomDesktop. If you want to switch themes without reloading, or if you want to switch themes temporarily (until page reload), this script is still helpful.