There are literally hundreds maybe thousands of WordPress themes available on the Internet which you can download and use; often all that is required is that you leave the source information in place so that you can get and use the theme for free.
That said there is often a desire to modify the theme further, to give it your own look and style. If you have coding skills then you can access the code screens and start to make the changes you think you would like. A word of advice though make sure you have created a back up before starting (as stated in the video) so that you can revert if it all goes wrong. Also remember that some browsers will present the code differently to others, so when you are making changes ensure that you check the results in multiple browsers; IE and Mozilla Firefox as a bare minimum.
The video below describes how you go about editing the theme using the css module via the theme editor module.
If you are not technically up to speed with coding and you are not sure how to go about it, even with the help from the video tutorial, you can opt for a non technical WP Remix software package. There are costs involved with doing this but you can at least get the theme you want without taking an age to learn the technical skills required to do it through coding. Check out the WP REMIX Review to find out more detail. Or just click on the WPRemix banner in the sidebar to go straight to site.