Selecting a style is also available to users within their Preference Editor.
1) Profile Dropdown Menu (Member Name/Top-Right)
2) Click Preferences
3) Select the Preferred Forum Style (aka Skin/Theme)
I'm sure that if you only had two styles, you could create a link to hard-code toggle between style named "DEFAULT" and "NOT-DEFAULT," so that you'd have a "dumb toggle."
This would be something you'd have to custom program yourself, since there are plenty of sites that allow users the choice between more than just 1/ DEFAULT and 2/ NOT-DEFAULT.
You could even relocate the drop-down to another location on your pages, so that its right up-front for the user to click and toggle between. Then you can populate it with all the styles you want -- or just two. But that would add clutter to the most important parts of your pages. Really how often do users toggle between multiple styles on each visit session?
But the very simple alternative would be to just offer up the end user a browser-extension.
I recommend DARK READER: https://darkreader.org/help/en/
Dark Reader for Chromehttps://chrome.google.com/webstore/detail/dark-reader/eimadpbcbfnmbkopoojfekhnkhdbieeh
Dark Reader for Firefoxhttps://addons.mozilla.org/en-US/firefox/addon/darkreader/