The wiki’s CSS affects the experience for every user in the wiki, and can only be edited to by Admins.

Your personal CSS only effects your experience and can only be edited to by you

Thank you.

What Bladdest said. It's also important to note that some scripts can only be on your personal CSS due to violating the Customization policy if they're wiki-wide.

I just wish to test out my username colors on the WoF wiki for the WCFW wiki

Is there a way to make the discussions show username colors?

Custom CSS doesn't work in Discussions.

For the wiki-wide

Wiki-wide or not, you just can't change username colors on Discussions. This is a limitation in Fandom itself.

Do any of you know a way to make it one color for light mode but a different one for dark mode? I saw it on a wiki once

.theme-fandomdesktop-dark 
{
WHAT YOU WANT TO APPEAR IN DARK MODE
}

.theme-fandomdesktop-light
{
WHAT YOU WANT TO APPEAR IN LIGHT MODE
}

In the wiki’s CSS, iirc.