This guide will let you know how to Apply Themes to your Steam Profile.
First, you have to open the Steam client, then go to your profile and click on the Edit Profile button. After entering your profile, you need to go to the themes tab on the left-hand side of your screen. Scroll down and you will find six themes. You can select any one of them by clicking on them.
After selecting the theme, you have to click on the Save button on the bottom right of your screen. Then go back to your profile and you will find this theme in your profile.