By default, Theme My Login includes a minimal stylesheet, titled theme-my-login.css, from the plugin directory. In most cases, this is all the styling needed for a decent looking log in form. However, some themes may "mess it up" or you may just need something more customized. This is where the custom stylesheet comes in to play. If you create a copy of theme-my-login.css in your theme’s directory, this version will be loaded instead of the default version. This is a great way to maintain your customizations between plugin upgrades.
Theme My Login uses most of the same selectors as the default login page. However, for ease of targeting, all Theme My Login forms are wrapper in a DIV with a class of "login" applied. This same DIV also has an ID of "theme-my-login" with the instance appended to it. This is particularly useful when using the template tag or shortcode, as you can assign your own instance with the "instance" argument. You can then use that instance ID to target that particular form.