The easiest and quickest way to change the look of your site is to use the inbuilt themes that Google have included.
First, you click on the more button on the top right of your site, then choose the "Manage site" option.
Down the left hand side of this screen is a list of options. You need the last one, "Themes".
The screen you will see next is a selection of different colours and themes.
You can use the preview button to see what they would look like on your site.
When you are happy, click the save button.
The theme used on this site is "Crystal Panel".
For this, you will be using the following online tools :-