Maximalist UI's

(Is there some open source media player with this kind of skins?)

The OG winamp is still available and fully working
ew windows media player
Winamp has been open sourced github.com/alexfreud/winamp
GitHub - alexfreud/winamp: Iconic media player

Iconic media player. Contribute to alexfreud/winamp development by creating an account on GitHub.

GitHub
Read their license. It does not meet the definition of open source.
The Open Source Definition - Open Source Initiative

Introduction Open source doesn’t just mean access to the source code. The distribution terms of open source software must comply with the following criteria: 1. Free Redistribution The license shall...

Open Source Initiative

Wow, they prohibit forking and distributing changes, that fucking blows.

Also, doesn’t look like it supports Linux natively, which is a shame.

skins.webamp.org

Skin museum

Fun trip down memory lane

Winamp Skin Museum

Infinite scroll through 100k Winamp skins with interactive preview

Winamp Skin Museum
XMMS is able to use Winamp skins. This one seems to be WMP, but that one copied the concept of crazy skins from Winamp.
Qmmp might fit the bill, I apply old winamp skins to it for a nostalgia fix