I'm not quite sure I understand your request entirely sorry, do forgive me if I have misinterpreted something.
First up the location of things, the HTML for the login screen is separated into two parts.
The standard HTML, the bits shown on every page including the header, footer, and blocks is generated by the core_renderer in lib/outputrenderers.php and can be overridden by a theme renderer. You can find information about that on this tutorial here http://docs.moodle.org/dev/Themes_2.0_overriding_a_renderer.
Although I don't think that is the part you are interested in.
The HTML for the actual login page forms etc is in login/index_form.html its an old file that really needs to be updated. There is an issue to convert it to a renderer so that theme's can override it but it is yet to be done.
For the time being if you wanted to change the HTML there the only option would be to hack that file and make the changes you wanted.
As for the CSS for the login page it really depends upon the theme you are using and which parents it inherits from.
I would really suggest you don't make changes to core themes and instead create your own theme based upon the theme you want to use, create a CSS file for it and add what ever CSS you want to that.
That way you could use firebug or what ever other tool you want to create the specific CSS you want to alter the login page.
Probably the best thing to do would be to have read through the themes forum on Moodle.org and post any question you have there.
The tracker is a good place to report bugs but its the wrong place to get help customising Moodle. There are only a handful of developers where as there are thousands of participants in the forums and they'll be able to help you out much faster.
The other place to take a look if you haven't already is the theme docs for Moodle 2. They contain a lot of useful information about getting started with themes.
Check out the see also section.
If there is an actual bug with one of the theme's or Moodle please do let us know otherwise this issue is likely to be closed shortly.