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.

Where do I put in the CSS? Here’s my code:

  a[href$="/wiki/User:Flurry%5Ean%5EIceWing%5ELibrarian"],
 a[href$="/wiki/User:Flurry%5Ean%5EIceWing%5ELibrarian"] {
    color: #000;
    font-weight: bold;
    background: -webkit-linear-gradient(330deg, #EDF2FB, #E2EAFC, #D7E3FC, #CCDBFD, #C1D3FE, #B6CCFE, #ABC4FF); -webkit-background-clip: text; -webkit-text-fill-color: transparent; }

Is that the css you want to have shown in light mode or dark mode?