If you're knowledgable about CSS you could use the Stylish extension to customize websites, but given frutiger aero's aesthetic, you'd probably have to add a bunch of images for ui elements, so I wouldn't neccisarily expect it to be easy
Short of customizing stuff yourself, I think thats a really niche styling to be looking for, for already niche platforms. If you wanna check for other people's CSS creations you can go to the userstyles.world site, or the older and I think now defunct userstyles.org, and search for mastodon, or any other sites you wanna check out themes for. There may not be many, customizing your websites with CSS stylesheets isn't exactly a huge hobby, so the most themes are available for sites everyone uses (YouTube, Wikipedia, google, etc.). Any themes there you find could be used with the stylish extension, though it may require some extra work from the older site
As someone else who enjoys the y2k retro aesthetic, I hope you find something you enjoy :)