https://www.adium.im/screenshots/images/overview.jpg
The area inside the chat window is HTML rendered via WebKit and you can change the theme easily. The Contacts list is also HTML in the middle so that's all theme-able too.
https://images.six.betanews.com/screenshots/1110403338-1.jpg
Here's a Contacts list that's more compact and a chat window without the top toolbar.
https://adium.im/screenshots/images/overvieworange.jpg
Here's a more radical theming where they've gotten rid of the window around the contact list so it's floating.
If you go through the Adium Xtras, you can see all sorts of chat (https://www.adiumxtras.com/index.php?a=search&cat_id=5&sort=...), contact list (https://www.adiumxtras.com/index.php?a=cats&cat_id=4&sort=do...), and other ways of styling Adium.
KDE's Kopete (and after that, the now defunct KDE Telepathy) could that too. Maybe GNOME's Empathy client could that too. I agree all of that was really cool.
I sorely miss those all-in-one messengers completely integrated to the desktop.
(I started that project decades ago and still grateful to it, as it was the start of my career in tech)
On top of that, Adium is/was also incredibly themable. You could use independent Adium Xtra addons for both your contact list and your chats. I had a theme so that all messages just appeared sequentially like so:
[Hello][Hello][What's Up?][Etc.]
where mine were red and the person I was talking to was in blue (or vice versa, it's been awhile). You could see so many messages at once.
Adium is the application I miss most from MacOS, although I imagine I'd still miss it even if I were still using a Mac. With the exception of the addition of multi-device continuous chat history, we've really regressed in the category of instant messaging since the days of multi-chat clients.
I still have it installed. It was very useful in the PowerPC era.
Pidgin still is useful: it talks to a whole bunch of services that are still alive... IRC, Telegram, Skype, Google Hangouts, Slack, Rocket.chat, XMPP, ICQ, etc.
Sadly Adium can't use Pidgin or Libpurple plugins directly. This severely reduces its usefulness today.