Comment modifier le CSS d'un thème d'essai Wp

Bonjour,

J'ai un site Wordpress depuis quelques mois avec un "thème d'essai", thème gratuit durant un petite période. Maintenant que celui-ci est payant sur la toile, impossible de changer l’aspect graphique du site
http://www.bancdemusculation.biz/musculation-et-sante/
Comme vous pouvez le voir, il n'existe pas plus sobre comme page actu ...

Voila le texte indiqué dans l'éditeur
"Layers is a revolutionary new way to create websites with WordPress. Now you can create beautiful, responsive pages faster and easier than ever. Whether you're a pro or have never used WordPress before, Layers will get you up to speed quickly and give you a jump-start on your layout with pre-existing templates or a blank page, ready for your content. With a simple point-and-click editor built right into the WordPress customizer, you simply select what you want to change, change it, and you're done. You're always in control, and always working with a live, front-end view of your website. Pages are built by stacking content widgets together. The unique and powerful Layers Design Bar connected to each widget is where the magic happens. Here you choose the options for the display of your content. It's so simple and intuitive, you'll be a pro the first time you use it. Layers reacts to the content you give it, generating balanced, mobile-ready layouts without requiring a line of code. Layers is free, and always will be and you can take Layers even further with quality child themes and extensions which give you more functionality and more ways to display your content.
Layers is WordPress made easier."

J'ai besoin d'aide !! =)