To select a new theme:
Go to Admin > Site Settings > Customize Your Site > Themes and Images > Color Themes
To change the colors of key elements:
Go to Admin > Site Settings > Customize Your Site > Themes and Images > Customize Theme
Review the look and feel of your changes
If you have customized colors on your site, be sure to review all pages and forms to ensure that critical elements aren't hidden or unreadable because the font color is similar to the background color.
Advanced customization options for web designers:
Go to Admin > Site Settings > Customize Your Site > Themes and Images > Advanced:
If you are a web designer with CSS skills, you can write your own stylesheet, and make the site look exactly the way you want it to. NOTE: this is an expert-level feature, and we do not provide support to designers wishing to write their own CSS (but if your attempts go awry, you can simply delete any CSS entered above to revert to the default look and feel).