To find the code that could help you, you will have to search the internet and learn CSS if you don't know it. To find the CSS class or ID which you have to apply your CSS to, you will have to use your browser developers tools.
If you don't want to use css you can use a seperate list thet represents your lead source and subscribe your user automaticly to that list.
And if you have the Enterprise version you can setup a daily Automation that will set a value for your custom lead source field for all those users who are subscribed to this extra list and then unsubscribe them from this extra list. You set it up once and it will perform this task automaticly for you. Ain't that nice?