Within your DotNetNuke installation, go to a page that displays the log in module.
Use the web developer toolbar, go to the information menu / display ID and class details.
This will display the IDs for each of the elements on your page, from here you can identify the IDs that are used for styling the log in module.
From here you can add CSS code to your skin.CSS file which targets those IDs and you can add the styling that you require.
I have explained in detail how to use the web developers toolbar in this tutorial:
Troubleshooting DotNetNuke Skins