Assuming the desktop is GNOME is a bold assumption, AFAIK there's no gnome under OSX nor windows, and personally GNOME has displayed so much bad faith and disdain for the need of its users that even if it were the best desktop available I would not touch it with a 10 foot stick. Also GTK applications usually do not look bad under KDE or LXDE or cinnamon, etc.
That being said the official screenshots are horribly dated, maybe more than 10 years. They feature version 2.2 while current version is 3.5.1 better than words here a screenshot of sylpheed 3.5.1 under a plasma 5 desktop: https://framapic.org/W9q8F6IAArBR/6aCKA6Wpk87V.png
I tried a variety of email client and none have been fully satisfying, evolution was really painful to use and half-broken most of the time, it's been a while since I've used thunderbird and though it tends to work I have no plan to go back until it offers the possibility to prevent access with a password as the mozilla suite offered. Mutt was nice but I moved away for personal preferences. Right now I use kmail which I recommend you do NOT use, far from being the pain evolution was, it mostly work most of the time but when it does not you're in for a ride, the main drawback is the akonadi backend that offers nothing I want or need and gives me headache. every other day I have stop and restart akonadi so the new mails would show in my inbox, once in a while it fails silently to send email, the documentation is sparse and the bug reports are scary[1] and the fix you may finally find are not obvious[2]. I had hopes in trojita but last time I checked it was still not ready for daily use after years of development. I had even better hopes for caliopen[3] but current roadmap says late 2018.
The good news is KDE has been working on replacing akonadi by Sink which is supposed to have learned from their mistakes and address the weak points of akonadi , and replacing kmail with kube which will leverage Sink and be consistent with roundcube next. Bad news is they've been at it for a couple years now and it's not done yet.
Next in line for me to give a try are elementary OS' mail[4] and notmuch in emacs (also exists for vim).
[1]: https://bugs.kde.org/buglist.cgi?component=general&order=cha...
[2]: http://www.dvratil.cz/2017/01/kmail-multiple-merge-candidate...
[3]: https://www.caliopen.org/en/
[4]: https://elementary.io/#showcase-mail