According to the linked post, the link color and text color on your wiki just need to have a 1.5 contrast ratio. You can use an online contrast checker to check your ratios (I usually use WebAIM).

For reference:

• Text color on light theme: #3a3a3a

• Text color on dark theme: #e6e6e6

For example, if your link color on the wiki's dark theme is #ffcc00 and you check it against #e6e6e6, it's going to fail with a 1.2:1 ratio, so the links will be underlined. I have had to change the link color on one of my wikis too, and as long as the ratio is at least 1.5:1, the underline should disappear.

Hope this helps!

@Alytta But the colors are already inversed in Dark Mode, so they are clearly readable.

The underline specifically shows up when the contrast ratio between the text and link colors are low, regardless of how good the contrast is against the page background.

Community Central's current light and dark themes also underline their links (see this page for an example), even though the contrast looks good enough to the naked eye.

But when you compare the actual ratios of the text and link colors, you'll find the contrast is too low:

• Light theme: #9b004e -> #3a3a3a = 1.35:1

• Dark theme: #ffc500 -> #e6e6e6 = 1.27:1

If you're having issues with finding colors that work well in Theme Designer, you can contact Fandom at Special:Contact and give them feedback and hopefully some more improvements can be made.

I finally got the underlines to stop showing. Thanks!