Changing themes is easy and fun to do. You can change the theme to see immediately how your website would look in the given theme.
Go to Design Editor > Themes
Select a theme
When you found a theme that suits you best click Publish Your Changes
Here are some important things to note:
You don't have to change your content– it will transfer between themes without having to do anything.
You'll have to choose colors and fonts again because color categories vary per theme, your color/font choices don't switch between themes.
If you have image(s) set as your home, after switching themes check to see if they need to be resized. If you have video, there is no need to do this. If the image needs to be resized it will look like this:
To resize just hover over the image and click Crop & Edit.
If you have custom CSS or HTML you have to double check that it's still applicable to the theme you choose.