With this theme the widget title has a grey bar next to it which is achieved with a bg image on the parent div. This is why it is 18px. All other is sized in ’em’ units which are much preferred in a responsive theme.
e.g. to change title on a single post page
If you use the Google Chrome browser you can always right click any elements and select inspect element. This will help you find the css selector you need and also show which styles are currently applied to that element. Other browsers have similar options.