Change default CSS of Google Chrome

I would like to be able to change the default CSS used by chrome, such as removing the underlines for links, etc. However, I cannot find the default CSS file used by Chrome. Does anyone know where it is?

(Update 2014) Since support of user stylesheets was recently removed from Google Chrome, the only option for this moment is to use extensions (like Stylus *1), but these will behave differently (see below).

The most relevant request for re-introducing true user stylesheets in Google Chrome is Issue 347016: Support user stylesheets.

Per specification, "true user stylesheet" rule has lower origin specificity in cascade than author rule, but !important user stylesheet rules have higher origin specificity than !important author rule, regardless of its selector specificity.

Extensions mimicking user style-sheets in Chrome simply injects (hopefully) last style element into page, what have some consequences:

• such style is at "author level" in the cascade, so you must make sure your !important rule has higher specificity than one you want to override
• "your" injected style is exposed to page scripts, so they can easily delete it at will.

*1 Original extension, Stylish, is currently (2017) in state of erratic development by a new mainetainer, so I'd advise to avoid it and use alternative like aforementioned Stylus.

(Following original answer is now obsolete.)

Afaik it is not possible to tweak the UA css itself, but you can make a global userstyle: start Chrome once with --enable-user-stylesheet. This will create <user-data-dir>/<profile>/User StyleSheet/Custom.css which you can use (changes are propagated immediately). http://code.google.com/p/chromium/issues/detail?id=2393

You can have a look at this discussion : Issue 2393: Support user stylesheet

At the very end they mention that the --enable-user-stylesheet parameter when starting Chrome would enable custom stylesheets..