This depends on a few things .
In my case I have a rocket theme template and I edit the template.css file
under my /templates/rt_panacea_j15/css/template.css file directory
Best way is to get firebug or webdeveloper and style it live
( I find )
As for heights and widths etc also depends on templates and setup i think
as im not sure , but it looks to me like dt reg uses the joomla form fields
and not its own ( As I can edit them via my template )
Not much help I know but maybe worth a look via firebug to see what I mean
One of my edits was in template.css that i could style the fields
#rt-main .inputbox, #rt-main input#email, #rt-main input#name, #rt-main input#username, #rt-main input#password, #rt-main input#password2, #rt-main input#passwd {
border: 3px solid green;
background: #000;
color:#fff;
}
Just for example.
[attachment=0:3hx4b1so]<!-- ia0 -->form.jpg<!-- ia0 -->[/attachment:3hx4b1so]
also viewing source of the page is always good
You can see the id's and classes used
for example
Changing the asterix color ( Required )
If you view the source of your form you can find this
<span class='dtrequired'> * </span>
Which tells you that the class is dtrequired
You can use firebug to find where this class is ( In Firefox of course )
so we find its in /components/com_dtregister/assets/css/main.css
So you can edit the color size etc ( Or input your own styling )
So styling can be done via css with the right tools
http://getfirebug.com/downloads