Themes are the integral part of the look and feel of your site. As discussed they will determine your menu structure and make your site ‘feel’ the way you want it to.
If you think about a website like a cell phone, the theme is how the apps etc. look. The plugins (which we will discuss) make the website behave how you want it to.
Restaurant Example : https://pixelgrade.com/themes/rosa/preview/?market_ref=envato
Lawyer Example : http://digitallaw.thememountdemo.com/themeselector/