Maybe you are trying to use a font that is not installed on your wiki. But to check, you should give the link to the page you are talking about.
font-family: 'Gentium Book Basic', Georgia, Palatino, 'Palatino Linotype', Times, 'Times New Roman', serif;
}
This is what I'm trying to do. And I'm wanting to install Caslon Antique and the italic Caslon Antique.
For the Gentim Book Basic font, put this code at the very beginning of your wiki MediaWiki:wikia.css page:
@import url('fonts.googleapis.com/css2?family=Gentium+Book+Basic&display=swap');
The other fonts you are mentioned, as well as the Palatino fonts, seem to not be free, but maybe someone else here knows better. There are other free Caslon fonts here.
By the @import statement I mentioned in the previous message, but only if it's free to use.
You can import other google fonts the same way.
Importing from Google Fonts is easiest since you can do it using @import as HumansCanWinElves just described. For a more in-depth explanation of what is going on, see Help:How to import Google Fonts. Google Fonts is not the only source of free-use fonts. However, other sites have a mix of free-use and paid-use fonts so you would need to be very careful about which fonts you use. Additionally, most other font sites do not allow you to import directly in CSS via a URL. Most of them require you to download the font file, upload it to the wiki, and then use @font-face to define the font within your CSS. That process is more complicated especially as the default MediaWiki configuration does not allow font file to be uploaded. Wikis that need to upload such files need to contact staff and get them to enable support for uploading such files.