Is there a way to change the GUI of notepad++? For example, is it possible to change the tab bar of notepad++ into something that looks like Chrome's tab bar?
Is there a website dedicated to notepad++ color theme? Is there a plugin for it? And is there a Github-like color theme for notepad++?
You can't change the application theme, however you can change toolbar for a start. Here's the toolbar with a set of tango icons;
There's a really interesting blog post (PaintBits - Improving Notepad++ into the perfect IDE), and a section on the NpWiki++ - Can I customise the toolbar? - both detailing how to customise Notepad++'s toolbar.
Theoretically you could re-purpose the following images to create your desired Chrome like toolbar;
As for tabs, their colour and font style can only be set;
Sadly there doesn't seem to an effort to bring all UI style configuration settings (toolbars, tabs, etc) under one setting (therefore no theme website you asked about). Perhaps suggest something on the Notepad++ Idea Tracker