Username Check module provides us a very user friendly autocomplete form to check if user entered username already exists or not.

The only problem that likely to occurs on your site with custom theme is the positioning of the ajax throbber image.

Since the module "calculates" the position of the image using java script, then practically overriding the css for the image position is kinda useless.

The approach to handle this problem is by creating another java script to move the image after the original script positioning.

To do that you can use this code :

 
$('#username-check-informer:not(YOURTHEMENAME-processed)').css({left: '662px', top: '585px'}).show().addClass('YOURTHEMENAME-processed'); 

in your custom theme script.js.

note you need to find out the correct left and top position, It can be done easily using firefox firebug extension.