Every company has its own corporate identity and therefore changes the look and feel of SharePoint site. Themes allow a basic customization based on color, fonts and images while not changing functionality.
You don't need knowledge about SharePoint development. With site owner or designer rights you can change the site theme.
How to change site theme in SharePoint
Changing the site theme offers different functionality depending of your SharePoint version. Go to site settings and site theme to change it. You can select a new theme and change colors e.g. for links. You can also preview the new site theme before applying it (not available in Foundation).