Yes read the error message, it is about the colour used that is not valid.
Follow the weblink that the error provides and you will see a link to the 140 redefined colors that are approved for use on the web for the text and buttons as in ‘white’ etc.
https://www.wix.com/corvid/reference/$w.Style.html#color
https://www.w3schools.com/colors/colors_names.asp
Also, as for the background container colour itself, #323232 is a very dark gray which is nearly black .
https://www.colorhexa.com/323232
However the nearest actual WEB SAFE colour is #333333
https://www.colorhexa.com/333333
You also don’t need to bother with the #FF in front of the hexidecimal rgb color value either, even though it shows that in the Wix API Reference link.
If you do actually put the colour value as #FF… then all it does is make the background completely black when you flip the switch regardless of whatever colour you used for teh background.
I have opened up the tutorial myself and tried it using many other different hexidecimal rgb color values and it all works fine.
Also, have a read of the post about the original tutorial as well.
https://www.wix.com/corvid/forum/corvid-tips-and-updates/example-dark-mode-switch