When you first built your website, you already have a theme chosen to start. But as we get newbies and even some that's been here at WA for a bit, sometimes need reminding where things are located (like myself)(in my 5th Premium month) LOL!. So, I'm just trying to help out a little bit to try and make things a little easier. That's my hopes anyways.
How to get to Themes in your back office:
On your WA feed page to the left, click on websites and a pop up will pop up (LOL) as shown below:
Find the website you want to change the theme to, found to the right in the image above and click log in.
Once you are logged in, you are in your back office of your website. Now, we are looking for themes. Let's go!
Hover over the word appearances in the left hand column and a little pop up will pop up (LOL) and you will see themes as shown below:
Click on Themes!