To customize your Site's appearance, first, click on "Website":
From here you'll click on "Appearance" in the sidebar. If you're happy with your Site theme, but want to change the fine details (colors, text, imagery, etc.), click the "Customize" button on the Current Theme:
You'll then be brought to the Theme Editor. On the left will be a sidebar labeled "Settings". This is where you'll make all your changes! (Note: Settings are different for each theme, so don't worry if they don't look exactly like this!). At the left of the footer is a Mobile / Desktop view switcher, so you can see what your Site looks like on a mobile device. In the top right there is a Current Page selector, allowing you to navigate the different pages in your Site (The Current Page selector will display the first instance of the selected page. For example, the Sales Page selection will display the first Sales Page in the Sales Pages section of your Website.)
You can change your Settings either by navigating through the Settings sidebar on the left, or by clicking on the specific element you wish to change. For example, if I want to change the "Purchase Offer" text on my Sales Page, I'll just click the element itself:
This will automatically navigate you to the specific setting in the Settings sidebar:
Have a bunch of fun customizing your Site's appearance to your heart's content!