So you need to customize the look of a Joomla module? This is quite simple, just follow these steps:
1) Go to the backend -> Extensions -> Module Manager -> Find your module and set the name of the custom css that you will add
2) Refresh the page and check the source code. You will find the element using the new class name as shown below
3) Modify the css file and include the specific propreties you want for this module and use the name set previously
That's it, now you can make your modules look the way you want!