2
The following is for 2.0.13.2 (and most recent precedents) and for 2.2.3a (I haven't checked 2.2.4 but I assume it's the same):
It depends on what you're trying to 'adjust' - part of the styles for this are defined in the template (/modules/system/templates/system_uerform.html). The rest is controlled in /modules/system/style.css